The US-led coalition said Wednesday it has doubled to about 450 the number of military advisors assisting Iraqi troops engaged in the fight to retake Mosul from Islamic State fighters.

"We have increased the number of advise and assist forces that are there with the ISF (Iraqi Security Forces) command elements to help advise them as they move forward and to synchronize operations," coalition spokesman Colonel John Dorrian said in a video conference from Baghdad.

Dorrian said the reinforcements were part of a series of measures taken to "accelerate the advance of the Iraqi security forces."

While military advisors are behind the frontlines, they have already entered the city several times, he added.

American military forces are carrying out air and artillery strikes in Iraq as part of a US-led coalition against the Islamic State group, and have provided training, advice and other assistance to Baghdad's forces.

There are about 5,000 American military personnel in Iraq, according to the coalition, and US special forces personnel have also fought IS on the ground.

More than 125,000 Iraqis have been displaced since the Mosul offensive began in mid-October, according to the United Nations. More than 3.3 million people are currently displaced in all of Iraq.

A senior commander from the elite Counter-Terrorism Service that has done most of the front-line fighting inside the city told AFP on Sunday that Iraqi forces now controlled more than 60 percent of Mosul's eastern half.

"There are more than 200,000 buildings in Mosul and ... you end up having to clear each one," Dorrian said.

"And that goes from rooftop level, often in four-story or higher buildings, through every single room, and every single closet, and into tunnels that have been dug between these buildings, and sometimes beneath them," he added.

"It's going to take time. It's going to be extraordinarily dangerous."

Iraqi forces have yet to enter west Mosul, which is still completely held by IS fighters.

"There's been a lot of discussion about the losses that Iraqis are taking -- the enemy is taking an order of magnitude greater," Dorrian said.

Officials say it could be months before Iraqi forces are able to completely retake Iraq's second city, where hundreds of thousands of civilians still live.

Some were forced to stay by the Islamic State group, others remained for fear of losing their property, because winter conditions in displacement camps are harsh or simply because escape routes are not safe enough.