IS has lost the ability to move in large convoys, making it more difficult to replace fighters if it loses them in significant numbers
"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 location.
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.
IS has lost the ability to move in large convoys, making it more difficult to replace fighters if it loses them in significant numbers.
But the US-led coalition against IS has said that the jihadists can still travel in smaller groups.
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.