(CNN) American warplanes hit an ISIS camp in Libya where foreign fighters had been engaged in advanced training, White House press secretary Josh Earnest said. Local officials said at least 49 people had been killed and six injured.

Noureddine Chouchane, a senior operative in the terrorist group from Tunisia, was believed to be among those from around Africa and the Middle East who had converged on the site. It was not immediately clear whether Chouchane was killed, Earnest said Friday.

Chouchane is thought to have played an instrumental role in two terrorist attacks in Tunisia last year, one at Tunis' Bardo Museum that killed 23 people and another at a seaside resort in Sousse that left 38 people dead. ISIS claimed responsibility for both massacres.

Conflicting information emerged about whether the strike also might have killed two Serbian Embassy employees who were kidnapped last year.

Serbian Foreign Minister Ivica Dacic said the pair -- communications officer Sladjana Stankovic and driver Jovica Stepic, who were kidnapped in Sabratah in November 2015 -- were believed to be among the dead.

The Pentagon knows of the reports claiming deaths of "two Serbian hostages," but "at this time, we have no information indicating that their deaths were a result of the strike," spokesman Peter Cook said.

"Our forces watched this training camp for weeks leading up to the operation, and at the time of the strike there were no indications of any civilians present. While the circumstances of their deaths remain unclear, we, nevertheless, express our deepest condolences to the Serbian government and the families of those killed," Cook said.

"When conducting our operations, the U.S. military goes to extraordinary lengths to limit the risk of civilian casualties, and in our campaign to defeat ISIL we will continue to do so," Cook said.

A spokeswoman for the Serbian Foreign Ministry emphasized that Serbia still was waiting for official confirmation from Libyan authorities about whether the Serbian pair were among the dead. She said that Serbia had been informed by the Pentagon that the specific location where they were believed to be held had been bombed.

Friday morning's U.S. strike in the al-Qasser district in Sabratha, a coastal city in northwestern Libya where most residents are from Tunisia, killed at least 49 people, Hussain al-Thawadi, the Mayor of Sabratah, said.

Al-Thawadi told Libya TV in an interview the death toll could rise because more people might still be under the rubble. He said the house was rented by suspected ISIS members, and he believed more than 60 people were inside it when it was hit.

Also six people were wounded, according to the Sabratha Municipal Council.

CNN Map

A Libyan man started to expand the house in Sabratha to several levels last year, security officials in the city said. He had brought in several groups of fighters over the past few months, including one batch two days ago. That house was struck Friday.

Over the last several weeks, the United States observed militants moving around the site and undergoing what appeared to be special training, a U.S. official said. "This was outside the normal training camp scenario," the official said.

JUST WATCHED Fears as ISIS expands into Libya Replay More Videos ... MUST WATCH Fears as ISIS expands into Libya 02:05

The activity raised concerns the people there might be planning to launch an external attack, though no details were discovered about where or when this might take place.

The U.S. military has launched hundreds of airstrikes against ISIS targets over the past two years. These have been concentrated in Iraq and Syria , where the Islamist extremist group has established its biggest foothold and has its de facto capital in Raqqa.

But Libya -- a North African nation that's been in turmoil and a hotbed for some militant groups since a 2011 revolution that toppled longtime dictator Moammar Gadhafi -- has been in its crosshairs as well.

ISIS expansion in turbulent Libya

ISIS has emerged as the world's top terrorist threat, having conducted or inspired about 70 attacks in 20 countries since declaring its caliphate in June 2014.

Photos: Gunmen attack Tunisian museum Photos: Gunmen attack Tunisian museum People pray at the entrance gate of the Bardo Museum in Tunis, Tunisia, on Wednesday, March 18. Earlier in the day, at least 23 people were killed -- most of them tourists -- when gunmen opened fire in the museum. Hide Caption 1 of 15 Photos: Gunmen attack Tunisian museum A person is taken away on a stretcher outside the museum. Hide Caption 2 of 15 Photos: Gunmen attack Tunisian museum Bullet holes are seen in a bus window near the museum. Hide Caption 3 of 15 Photos: Gunmen attack Tunisian museum A helicopter flies around the museum, which is housed in a 19th-century palace and describes itself as "a jewel of Tunisian heritage." Its exhibits showcase Tunisian art, culture and history. Hide Caption 4 of 15 Photos: Gunmen attack Tunisian museum Tourists are moved away from the scene of the shooting. Hide Caption 5 of 15 Photos: Gunmen attack Tunisian museum Tunisian President Beji Caid Essebsi leaves a Tunis hospital after visiting those injured in the attack. Hide Caption 6 of 15 Photos: Gunmen attack Tunisian museum Security forces patrol around the museum. Hide Caption 7 of 15 Photos: Gunmen attack Tunisian museum The museum is evacuated by authorities. There was a significant number of foreign tourists at the museum at the time of the attack -- something that's not surprising, given its prominence in Tunisia and the fact that at least two cruise ships were docked in Tunis. Hide Caption 8 of 15 Photos: Gunmen attack Tunisian museum A security officer holds his position near the museum. Hide Caption 9 of 15 Photos: Gunmen attack Tunisian museum Security forces secure the area around museum. Hide Caption 10 of 15 Photos: Gunmen attack Tunisian museum Police arrest a man they believed to be suspicious. Hide Caption 11 of 15 Photos: Gunmen attack Tunisian museum Police in plainclothes stop a vehicle as they secure the area around the museum. Hide Caption 12 of 15 Photos: Gunmen attack Tunisian museum Tunisian security forces secure the area around the museum. Hide Caption 13 of 15 Photos: Gunmen attack Tunisian museum Security forces take up position outside the museum. Hide Caption 14 of 15 Photos: Gunmen attack Tunisian museum Police take up position behind a car near the museum. Hide Caption 15 of 15

It is significant that ISIS leader Abu Bakr al-Baghdadi exerts more control over the ISIS branch in Libya than any other, according to a report late last year to the United Nations Security Council . This conclusion is in line with U.S. intelligence determinations that al-Baghdadi sees the relatively lawless, impoverished North African state as prime ground to enlarge his self-declared caliphate.

by taking over territory and exercising terror, as evidenced by The group has asserted itself in Libyaby taking over territory and exercising terror, as evidenced by its beheadings of Egyptian Coptic Christians about a year ago on a Libyan beach.

Libya has also been a base to train militants, devise plots and launch them in places like neighboring Tunisia, which has been considered the Arab Spring 's success story but has not been immune to the violence wrenching the region.

The Bardo Museum and Sousse beach attacks are gut-wrenching proof of that, not just because of the human carnage but also for their negative effects on a Tunisian economy that's long benefited from tourism.

JUST WATCHED Tunisian Prime Minister speaks to CNN Replay More Videos ... MUST WATCH Tunisian Prime Minister speaks to CNN 04:01

Both attacks were carried out by one terror cell whose members came from Tunisia, communicated with ISIS leaders in Syria and Iraq, and trained in Libya, a Western counterterrorism source said. That training happened near Sabratha, about 60 miles (100 kilometers) east of the Tunisian border where the Friday morning strike took place.

The same cell planned to attack France's diplomatic complex in Tunis, only to be thwarted as Tunisian security forces moved in, according to the source.

All of its roughly half dozen core members -- at least those not killed in attacks -- who were still in Tunisia are now in custody. But others involved in the plots may not be in the country, possibly finding refuge in their former training ground in Libya instead.

U.S. official called for 'decisive military action' in Libya

A few weeks ago, Obama's top military adviser talked about stepping up efforts to curtail ISIS specifically in the North African country.

Addressing reporters while traveling in Europe, Gen. Joseph Dunford, the Joint Chiefs of Staff chairman, said the United States wants to "take decisive military action" to "check" ISIS in Libya.