The United States said Thursday up to 900 Islamic State group jihadists have been killed in the offensive to retake Iraq’s Mosul, as camps around the city filled with fleeing civilians.

“Just in the operations over the last week and a half associated with Mosul, we estimate they’ve probably killed about 800-900 Islamic State fighters,” General Joseph Votel, who heads the US military’s Central Command, said in an interview with AFP, speaking from an undisclosed military base.

Iraqi security forces and Kurdish peshmerga fighters are pushing toward Mosul along several axes and have made relatively quick progress as they approach Mosul, Iraq’s second city.

The offensive, which began 10 days ago, has so far been concentrated in towns and villages around Mosul, and resistance may get heavier as Iraqi forces break through IS defences and enter the city itself.

Earlier US estimates had put the population of IS fighters in Mosul itself at between 3,500 and 5,000. Up to another 2,000 were thought to be in the broader Mosul region.

Votel cautioned it was hard to provide precise numbers as IS fighters move around the city and blend in with the local population.

The coalition has previously said that it “does not use a casualty count as a measure of effectiveness in the campaign to ultimately defeat (IS) in Iraq and Syria”.

Despite this assertion, such figures are periodically announced.

Votel said he had spoken with Iraqi military leaders late Tuesday who told him that, as of that time, 57 Iraqi security forces had been killed and around another 255 wounded.

For the Kurdish peshmerga, numbers were lower, with about 30 killed and between 70 and 100 wounded.

IS has lost the ability to move in large convoys, making it more difficult to replace fighters if it loses them in significant numbers.

Small groups of IS fighters have fled Mosul in recent days, Votel said, heading out of the city on the west side that has not been encircled by Iraqi forces.

Some strategists have called into question the Iraqis’ decision to leave part of the city open, but Votel said there were advantages to doing so.

“There are some reasons for allowing some exfiltrations. It certainly provides a way for the refugees of the population to get out of there,” the four-star general said, and “it does provide an opportunity to limit destruction in the city itself”.

The IS fighters that have fled so far are typically in small groups of four to six, and Votel said it is impossible to track them all.

“Nothing is 100 percent, but in this case I think we are doing a pretty good job,” he said.