Six Russian military planes have been intercepted off the coast of Alaska as they approached North American airspace.

US and Canadian fighter jets were scrambled to intercept the aircraft in two incidents on Wednesday and Thursday, according to military officials.

The Russian planes - two MiG-31 fighters, two Bear long-range bombers and two IL-78 refuelling tankers - had entered America's air defence identification zone but not crossed into US air space.

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Lt Col Michael Jazdyk, from the North American Aerospace Defence Command (NORAD), said the US jets intercepted the planes about 55 nautical miles from the Alaskan coast at 7pm local time on Wednesday.

He said the fighter jets were scrambled “basically to let those aircraft know that we see them and in case of a threat, to let them know we are there to protect our sovereign airspace”.

The Russian aircraft looped south and returned to their base in Russia after the American US F-22 planes approached.

Hours later, at 1.30am local time on Thursday, two Canadian CF-18 fighter jets were sent to intercept two of the long-range bombers about 40 nautical miles off the Canadian coastline in the Beaufort Sea.

Shape Created with Sketch. Ukraine crisis: A timeline of the conflict Show all 23 left Created with Sketch. right Created with Sketch. Shape Created with Sketch. Ukraine crisis: A timeline of the conflict 1/23 30 November 2013 Public support grows for the “Euromaidan” anti-government protesters in Kiev demonstrating against Yanukovych’s refusal to sign the EU Association Agreement as images of them injured by police crackdown spread. EPA 2/23 20 February 2014 Kiev sees its worst day of violence for almost 70 years as at least 88 people are killed in 48 hours, with uniformed snipers shooting at protesters from rooftops. AFP/Getty Images 3/23 22 February 2014 Yanukovych flees the country after protest leaders and politicians agree to form a new government and hold elections. The imprisoned former Prime Minister, Yulia Tymoshenko, is freed from prison and protesters take control of Presidential administration buildings, including Mr Yanukovych's residence. Genya Savilov/AFP/Getty Imageses 4/23 27 February 2014 Pro-Russian militias seize government buildings in Crimea and the new Ukrainian government vows to prevent the country breaking up as the Crimean Parliament sets a referendum on secession from Ukraine in May. Sean Gallup/Getty Images 5/23 16 March 2014 Crimea votes overwhelmingly to secede from Ukraine and join Russia in a ballot condemned by the US and Europe as illegal. Russian troops had moved into the peninsula weeks before after pro-Russian separatists occupied buildings. EPA 6/23 6 April 2014 Pro-Russian rebels seize government buildings in the eastern cities of Donetsk, Luhansk and Kharkiv, calling for a referendum on independence and claiming independent republic. Ukraine authorities regain control of Kharkiv buildings on 8 April after launching an “anti-terror operation” but the rest remain out of their control. EPA/PHOTOMIG 7/23 7 June 2014 Petro Poroshenko is sworn in as Ukraine's president, calling on separatists to lay down their arms and end the fighting and later orders the creation of humanitarian corridors, since violated, to allow civilians to flee war zones. Getty Images 8/23 27 June 2014 The EU signs an association agreement with Ukraine, along with Georgia and Moldova, eight months after protests over the abandonment of the deal sparked the crisis. LOUISA GOULIAMAKI/AFP/Getty Images 9/23 17 July 2014 Malaysian Airlines flight MH17 is shot down over eastern Ukraine, killing all 298 people on board. Ukrainian intelligence officials claim it was hit by rebels using a Buk surface-to-air launcher in an apparent accident. 10/23 22 August 2014 A Russian aid convoy of more than 100 lorries enters eastern Ukraine and makes drop in rebel-controlled Luhansk without Government permission, sparking allegations of a “direct violation of international law”. Reuters/Alexander Demianchuk 11/23 29 August 2014 Nato releases satellite images appearing to show Russian soldiers, artillery and armoured vehicles engaged in military operations in eastern Ukraine. Nato 12/23 8 September 2014 Russia warns that it could block flights through its airspace if the EU goes ahead with new sanctions over the ongoing crisis and conflict EPA 13/23 17 September 2014 Despite the cease-fire and a law passed by the Ukrainian parliament on Tuesday granting greater autonomy to rebel-held parts of the east, civilian casualties continued to rise, adding to the estimated 3,000 people killed Reuters 14/23 20 October 2014 A 'tactical missile' explosion at a factory sends blast wave across rebel-held cityof Donetsk YouTube 15/23 16 November 2014 The fragile ceasefire gives way to an increased wave of military activity as artillery fire continues to rock the eastern Ukraine's pro-Russian rebel bastion of Donetsk Getty 16/23 26 December 2014 A new round of ceasefire talks, scheduled on neutral ground in the Belariusian capital Minsk, are called off AFP/Getty 17/23 12 January 2015 Soldiers in Debaltseve were forced to prepare heavy defences around the city; despite a brief respite to the fighting in eastern Ukraine, hostilities in Donetsk resumed at a level not seen since September 2014 AFP/Getty 18/23 21 January 2015 13 people are killed during shelling of bus in the rebel-held city of Donetsk Reuters 19/23 24 January 2015 Ten people were killed after pro-Russian separatists bombarded the east Ukrainian port city of Mariupol AP 20/23 2 February 2015 There was a dangerous shift in tempo as rebels bolstered troop numbers against government forces AFP/Getty 21/23 11 February 2015 European leaders meet in Minsk and agree on a ceasefire in eastern Ukraine beginning on February 14. From left to right: Belarus President Alexander Lukashenko, Russian President Vladimir Putin, German Chancellor Angela Merkel, France's President Francois Hollande and Ukrainian President Petro Poroshenko. MAXIM MALINOVSKY | AFP | Getty Images 22/23 13 February 2015 Pro-Russian rebels in the city of Gorlivka, in the Donetsk region, fire missiles at Ukrainian forces in Debaltseve. Fighting continued in Debaltseve for a number of days after the Minsk ceasefire began. ANDREY BORODULIN | AFP | Getty Images 23/23 18 February 2015 Ukrainian soldiers repair the bullet-shattered windshield of their truck as their withdraw from the strategic town of Debaltseve. Following intense shelling from pro-Russian rebels, Ukrainian forces began to leave the town in the early hours of February 18. Brendan Hoffman | Getty Images 1/23 30 November 2013 Public support grows for the “Euromaidan” anti-government protesters in Kiev demonstrating against Yanukovych’s refusal to sign the EU Association Agreement as images of them injured by police crackdown spread. EPA 2/23 20 February 2014 Kiev sees its worst day of violence for almost 70 years as at least 88 people are killed in 48 hours, with uniformed snipers shooting at protesters from rooftops. AFP/Getty Images 3/23 22 February 2014 Yanukovych flees the country after protest leaders and politicians agree to form a new government and hold elections. The imprisoned former Prime Minister, Yulia Tymoshenko, is freed from prison and protesters take control of Presidential administration buildings, including Mr Yanukovych's residence. Genya Savilov/AFP/Getty Imageses 4/23 27 February 2014 Pro-Russian militias seize government buildings in Crimea and the new Ukrainian government vows to prevent the country breaking up as the Crimean Parliament sets a referendum on secession from Ukraine in May. Sean Gallup/Getty Images 5/23 16 March 2014 Crimea votes overwhelmingly to secede from Ukraine and join Russia in a ballot condemned by the US and Europe as illegal. Russian troops had moved into the peninsula weeks before after pro-Russian separatists occupied buildings. EPA 6/23 6 April 2014 Pro-Russian rebels seize government buildings in the eastern cities of Donetsk, Luhansk and Kharkiv, calling for a referendum on independence and claiming independent republic. Ukraine authorities regain control of Kharkiv buildings on 8 April after launching an “anti-terror operation” but the rest remain out of their control. EPA/PHOTOMIG 7/23 7 June 2014 Petro Poroshenko is sworn in as Ukraine's president, calling on separatists to lay down their arms and end the fighting and later orders the creation of humanitarian corridors, since violated, to allow civilians to flee war zones. Getty Images 8/23 27 June 2014 The EU signs an association agreement with Ukraine, along with Georgia and Moldova, eight months after protests over the abandonment of the deal sparked the crisis. LOUISA GOULIAMAKI/AFP/Getty Images 9/23 17 July 2014 Malaysian Airlines flight MH17 is shot down over eastern Ukraine, killing all 298 people on board. Ukrainian intelligence officials claim it was hit by rebels using a Buk surface-to-air launcher in an apparent accident. 10/23 22 August 2014 A Russian aid convoy of more than 100 lorries enters eastern Ukraine and makes drop in rebel-controlled Luhansk without Government permission, sparking allegations of a “direct violation of international law”. Reuters/Alexander Demianchuk 11/23 29 August 2014 Nato releases satellite images appearing to show Russian soldiers, artillery and armoured vehicles engaged in military operations in eastern Ukraine. Nato 12/23 8 September 2014 Russia warns that it could block flights through its airspace if the EU goes ahead with new sanctions over the ongoing crisis and conflict EPA 13/23 17 September 2014 Despite the cease-fire and a law passed by the Ukrainian parliament on Tuesday granting greater autonomy to rebel-held parts of the east, civilian casualties continued to rise, adding to the estimated 3,000 people killed Reuters 14/23 20 October 2014 A 'tactical missile' explosion at a factory sends blast wave across rebel-held cityof Donetsk YouTube 15/23 16 November 2014 The fragile ceasefire gives way to an increased wave of military activity as artillery fire continues to rock the eastern Ukraine's pro-Russian rebel bastion of Donetsk Getty 16/23 26 December 2014 A new round of ceasefire talks, scheduled on neutral ground in the Belariusian capital Minsk, are called off AFP/Getty 17/23 12 January 2015 Soldiers in Debaltseve were forced to prepare heavy defences around the city; despite a brief respite to the fighting in eastern Ukraine, hostilities in Donetsk resumed at a level not seen since September 2014 AFP/Getty 18/23 21 January 2015 13 people are killed during shelling of bus in the rebel-held city of Donetsk Reuters 19/23 24 January 2015 Ten people were killed after pro-Russian separatists bombarded the east Ukrainian port city of Mariupol AP 20/23 2 February 2015 There was a dangerous shift in tempo as rebels bolstered troop numbers against government forces AFP/Getty 21/23 11 February 2015 European leaders meet in Minsk and agree on a ceasefire in eastern Ukraine beginning on February 14. From left to right: Belarus President Alexander Lukashenko, Russian President Vladimir Putin, German Chancellor Angela Merkel, France's President Francois Hollande and Ukrainian President Petro Poroshenko. MAXIM MALINOVSKY | AFP | Getty Images 22/23 13 February 2015 Pro-Russian rebels in the city of Gorlivka, in the Donetsk region, fire missiles at Ukrainian forces in Debaltseve. Fighting continued in Debaltseve for a number of days after the Minsk ceasefire began. ANDREY BORODULIN | AFP | Getty Images 23/23 18 February 2015 Ukrainian soldiers repair the bullet-shattered windshield of their truck as their withdraw from the strategic town of Debaltseve. Following intense shelling from pro-Russian rebels, Ukrainian forces began to leave the town in the early hours of February 18. Brendan Hoffman | Getty Images

The air defence identification zone extends about 200 miles from the coastline, whereas air space stops 12 nautical miles from land, and officials said it is not rare for Russian planes to enter it.

In the past five years, jets under NORAD's command have intercepted more than 50 Russian bombers approaching North American airspace.

Also on Wednesday, the Swedish Foreign Ministry said two Russian military aircraft had crossed into its air space south of the Baltic Sea island of Oland.

The ministry called it a "serious violation" and said it has summoned the Russian ambassador over the incident.

According to Swedish newspaper Expressen, the planes were Sukhoi Su-24 combat jets that left Swedish air space when a Swedish Air Force fighter approached.

Additional reporting by agencies

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