Iraqi security forces will need help from "hundreds" of Western military trainers ahead of a planned assault to recapture the city of Mosul from Islamic State jihadists, a US official said Wednesday.

Mosul fell to IS fighters in June 2014 as they overran vast regions in northern and north-central Iraq, as well as in Syria.

Iraqi security forces who were supposed to secure the city collapsed in the face of the jihadist advance.

Following recent successes against the IS group, including the recapture of the Iraqi city of Ramadi by US-backed local forces, the Pentagon is now pushing Iraq to launch an assault on Mosul.

Colonel Steve Warren, a Baghdad-based spokesman for the US-led coalition that has been fighting the IS group since August 2014, said extra trainers were needed to increase the fighting capacity of Iraqi security forces.

"That's really the next step in generating the amount of combat power needed to liberate Mosul," he told Pentagon reporters in a video call.

"We know we will need more brigades to be trained, we'll need more troops trained in more specialties."

His remarks follow comments a day earlier from Pentagon chief Ashton Carter, who called on the US-led coalition to gather more trainers.

Warren said the number of extra trainers would be "certainly hundreds."

"There will be a US component to it," he said.

"Additionally, we want to see other partner nations, other members of this coalition, contribute as well.

"The secretary of defense said there are no free rides here. We expect everyone to step up and to contribute as best that they can."

Mosul, in northern Iraq, lies some 50 miles (80 kilometers) west of the Kurdish capital, Arbil.

The city holds special significance for the IS group, as it was where IS leader Abu Bakr al-Baghdadi proclaimed his "caliphate" straddling Iraq and Syria.

The United States currently already has about 3,500 troops in Iraq. Their mission is to train and advise local forces.

Another major Iraqi city, Ramadi, fell to IS fighters in May 2015 but local Iraqi forces -- backed by coalition air support and troop training -- recaptured the town at the end of last month in what was seen as a major blow for the jihadists.

Warren said troops were still clearing the city, which was littered with "thousands" of booby-traps, roadside bombs and houses rigged to explode on entry.

"It's a very complicated clearance process that goes on," he said.