Islamic State jihadists are using small commercial drones to attack Iraqi security forces in the battle for Mosul, a US commander said Wednesday.

Colonel Brett Sylvia, who commands an "advise and assist" US unit in Iraq, said IS fighters are attaching small munitions to quadcopters in an attempt to kill local forces as they retake Mosul, the last major IS bastion in Iraq.

"They are small drones with small munitions that they've been dropping," Sylvia said.

While the munitions were no larger than "a small little grenade," he said, that was enough to do what "Daesh does, and that's just, you know, indiscriminate killing," he said, using an Arabic acronym for IS.

The group's use of small drones is not new, Sylvia said, though initially they were mainly used for reconnaissance.

"They are (now) using them to drop munitions as Iraqi forces push into Mosul," he said.

He added that US-backed local troops have been able to bring down many of the drones, making them "much less effective than they were."

Tens of thousands of allied troops launched a huge offensive on October 17 to retake Mosul and areas around it.

Early in the offensive, a variety of forces quickly retook significant swaths of land, but the going has been much tougher inside the city itself.

Iraqi forces have retaken at least 80 percent of east Mosul from Islamic State jihadists, the spokesman of the special forces spearheading the campaign said Wednesday.

Over the past two weeks, Iraqi forces have overrun several districts and, for the first time, reached the Tigris River that runs through the heart of the city.

But the western part of Iraq's second city remains largely in IS hands.

"There's a lot of fight that's left to do in western Mosul," Sylvia said, noting that IS had conducted extensive defensive work.

Still, he said, IS resistance had weakened in several areas.