Story highlights There are 3,000 to 5,000 ISIS fighters in the Iraqi stronghold of Mosul

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(CNN) The US and its allies have killed between 800 and 900 ISIS fighters in the operation to retake Mosul, US Army Gen. Joseph Votel said Thursday.

US military officials estimate that there are 3,000 to 5,000 ISIS fighters defending the last major stronghold of the terror group in Iraq, and an additional 1,500 to 2,000 ISIS soldiers in a zone outside the city.

Votel is the head of US Central Command and is assisting the Iraqis in the fight for Mosul. A coalition of about 100,000 forces began the operation 10 days ago, and have been slowly gaining ground toward the city, liberating villages from ISIS rule along the way.

Clashes with ISIS fighters are expected to intensify the closer in troops get, and in Mosul itself bloody street-to-street skirmishes are expected, CNN has reported. ISIS is now sending "suicide squads" to Mosul from its symbolic capital of Raqqa in Syria, which the US and coalition forces also plan to try to capture soon.

ISIS executed a total of 232 people near Mosul on Wednesday, United Nations Human Rights spokesperson Ravina Shamdasani told CNN on Friday.

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