US Defence Secretary Ashton Carter held talks in Baghdad on Sunday to discuss the coming stages of the offensive to retake the city of Mosul from the Islamic State group.

Carter flew to the Iraqi capital on an announced visit to "survey key locations directly supporting the battle for Mosul," the Pentagon said in a statement.

The United States leads an international coalition providing assistance in the form of air strikes, equipment, training and advice on the ground to Iraqi forces battling the jihadists.

US forces are stationed in Qayyarah, the main staging base for the southern front of the Mosul offensive that was launched on October 17, as well as in the autonomous Kurdish region.

About 5,000 US troops are deployed in Iraq as part of an "advise and assist" mission to support Iraqi federal and Kurdish peshmerga forces battling jihadists.

US special forces are also active on the ground in Iraq, as well as in neighbouring Syria, where another offensive is under way to retake the other major remaining IS bastion of Raqa.

On Saturday, Carter told a security forum in Bahrain that Washington was sending 200 extra troops to join the 300 it has already deployed to support the Raqa campaign.

Carter met US troops, senior coalition commanders and Iraqi Prime Minister Haider al-Abadi in Baghdad, a coalition spokesman said.

He was also expected to hold meetings with Kurdish leader Massud Barzani during his visit to Iraq, possibly his last as US defence secretary.

Abadi had promised Mosul would be retaken by the end of 2016 but the going has been tough for Iraqi forces inside the densely populated city and commanders have warned the battle could go on for months.

- 90,000 displaced -

Most of the fighting inside Mosul, where hundreds of civilians still live, has been carried out by Iraq's elite Counter-Terrorism Service in the east of the city.

The army's 9th and 11th divisions have also moved in, while a myriad of forces are deployed on other fronts south, north and west but have not entered the city.

Top commanders told AFP on Saturday that federal police and elite interior ministry forces would soon join the army in southeast Mosul.

The Hashed al-Shaabi (Popular Mobilisation) paramilitary forces have been fighting on a western front, in a campaign aimed at retaking the town of Tal Afar and cutting IS supply lines between Mosul and Syria.

Hashed forces said on Sunday they had retaken four more villages near Tal Afar and southwest of Mosul, continuing an advance which has seen fierce desert battles in recent days.

The US-led coalition has launched hundreds of air strikes on Mosul and its surroundings in recent months but its action has been hampered by the continued presence of a large civilian population.

According to the United Nations, about 90,000 people have fled their homes since the start of the offensive.

"With the military operation moving deeper into the city, fighting in the densely populated parts of Mosul may trigger larger scale displacement," it said.

The latest humanitarian emergency compounds a years-old crisis which has seen more than 620,000 people forced to flee their homes in Iraq this year alone, bringing to 3.1 million the number of Iraqis currently displaced.

Displaced people in camps around Mosul have had to cope with subzero temperatures over the past week while conditions for those in Mosul were also dire, many without water or electricity.