Iraqi forces readied on Wednesday for an assault on Mosul airport after blitzing jihadist positions in a renewed offensive to retake the Islamic State group's emblematic stronghold.

Elite forces reinforced positions that were taken since a fresh push south of Mosul was launched on Sunday while hundreds of civilians fled newly recaptured villages.

"Around 480 people displaced from Al-Yarmuk area are being transferred to liberated areas further south," the federal police said.

Iraqi forces have retaken a key checkpoint on the main Baghdad highway south of Mosul and the village of Al-Buseif, a natural citadel overlooking the airport and the south of the city.

There were no major operations near Mosul on Wednesday, with Iraq's new interior minister visiting the village and the defence minister also expected on the front lines.

But Hashed al-Shaabi (Popular Mobilisation) paramilitary forces battled jihadists further west near the town of Tal Afar, which is between Mosul and the Syrian border and still held by IS.

The Hashed al-Shaabi said they blew up at least four car bombs in fighting near Ain al-Tallawi and killed several IS members.

The elite Counter-Terrorism Service (CTS) that retook east Mosul and did most of the fighting since the offensive on the city was launched on October 17 have not yet been brought into action in the latest push.

The interior ministry's Rapid Response units could also move in on the airport in the coming days, a key target before troops breach the city limits to face the jihadists in the narrow streets of Mosul's west bank.

- US forces 'under fire' -

Senior US officials this week estimated there were only 2,000 IS fighters defending west Mosul, suggesting the jihadist group had suffered heavy losses in the first four months of the operation.

The US-led coalition, which has provided intensive air support as well as advisers on the ground, said before the Mosul offensive began that 5,000 to 7,000 jihadists were in the city.

AFP reporters saw US forces moving into Al-Buseif on Wednesday in convoys of large military vehicles.

A coalition spokesman acknowledged later that US forces had been shot at and returned fire in the battle for Mosul.

"They have come under fire at different times, they have returned fire at different times, in and around Mosul," Colonel John Dorrian said, declining however to confirm if any US troops had been wounded.

The fate of an estimated 750,000 civilians trapped in west Mosul was a major source of concern as Iraqi forces prepared for what many have predicted could be one of the bloodiest battles yet in the war on IS.

Almost half of the remaining population are children, according to aid groups, and supplies are fast dwindling.

"Daesh fighters have seized all the hospitals and only they can get treated now," an employee at Al-Jamhuri hospital in west Mosul told AFP by phone, using an Arabic acronym for IS.

The health of many residents had been deteriorating for months under the rule of the "caliphate" which IS leader Abu Bakr al-Baghdadi proclaimed in a mosque near the same hospital nearly three years ago.

"Even before the hospitals were closed, locals had to pay Daesh sums of money they couldn't afford," the hospital employee said.

- IS threat -

Medical workers and residents speaking from west Mosul on condition of anonymity said the weakest were beginning to die of malnutrition and shortages of medicines.

Iraqi forces declared the full liberation of the city's eastern side a month ago but the situation there has remained precarious, with the departure of CTS to the western front leaving a security vacuum.

Around half a million civilians stayed on in east Mosul, making the screening process that would have been necessary to prevent IS members blending in with the rest of the population almost impossible.

Several attacks have already been carried out in "liberated" neighbourhoods and on Wednesday, some residents found threatening IS leaflets under their doors.

"Warning! To all residents and those present in the east side, you have to leave the city as fast as possible. Staying exposes you to death and you will be a legitimate target for the mujahideen," the leaflet said.

While more than 50,000 of the 220,000 people displaced during the first months of the offensive have returned to their homes, some people continued to flee from retaken areas for fear of IS reprisals.

The loss of Mosul for IS would effectively spell the end of its days as a land-holding force in Iraq, with only a few scattered and shrinking pockets of territory left.