The United Nations has said it is "racing against the clock" to prepare emergency aid for hundreds of thousands of endangered civilians in Mosul with an Iraqi army offensive looming to oust the Islamic State group from the western half of the city.

Earlier today Iraqi Prime Minister Haider al-Abadi confirmed government forces had taken complete control of eastern Mosul, 100 days after the start of the US-backed campaign to retake Iraq's second largest city from IS insurgents who seized it in 2014.

UN officials estimate 750,000 people remain in Mosul west of the Tigris River that flows through the last remaining major urban centre held by IS in Iraq, after a series of government counter-offensives in the country's north and west.

The west side could prove more complicated to take than the east as it is crisscrossed by streets too narrow for armoured vehicles, allowing militants to hide among civilians.

The Sunni Muslim jihadists are expected to put up a fierce fight as they are cornered in a shrinking area of Mosul.

"We are racing against the clock to prepare for this," UN humanitarian coordinator Lise Grande said.

Humanitarian agencies were setting up displaced people camps accessible from western Mosul and pre-positioning supplies in them, she said.

"The reports from inside western Mosul are distressing," she said in a separate statement. "Prices of basic food and supplies are soaring...Many families without income are eating only once a day. Others are being forced to burn furniture to stay warm."

Government forces have finished clearing the last eastern pocket held by militants - the northern suburb of Rashidiya, Major General Najm al-Jubbouri, commander of the northern front, told the local Mosuliya TV channel.

"I now call on these (armed forces) heroes to move quickly to free what remains of Mosul, especially the right (western) side," Mr Abadi told a news conference in Baghdad.

He also said new US President Donald Trump has sent messages offering to increase the level of assistance to Iraq.

Mr Trump has made the fight against IS a foreign policy priority.