Hundreds of American troops have arrived at Qayyarah air base, 40 miles south of Mosul, in preparation for an attempt to liberate the city.

The US forces will be providing support, logistics and supplies for an offensive intended to retake the last major city in Iraq held by Islamic State (Isis).

Pentagon press secretary Peter Cook told reporters on Thursday (15 September) that "the secretary of defense has made clear that our forces in Iraq are in harm's way. Everyone who is serving there is in a dangerous situation".

The latest reports suggest that the assault on Mosul may begin as early as October.

"Security forces are almost done with preparations for the fight and forces are being organised now, so it is very possible we will start the operation in early October," said General Maan al-Saedi, a commander in the Iraqi counter-terrorism force.

Massoud Barzani, the president of Iraq's Kurdistan Regional Government, has confirmed that the Iraqi army and Kurdish peshmerga troops will conduct the offensive.

"There have been multiple meetings between leaders of the Peshmerga and the Iraqi army. They have finally agreed on the military plan and the role of each side," Barzani said.

Qayyarah airbase has been under Iraqi control since July and is expected to be rebuilt to allow US and coalition air forces to take advantage of its position near Mosul, which was seized by Islamist forces in 2014.

"When the Iraqi Security Force is ready to move on in their operations to get after Mosul, we'll be prepared to support that and the airfield will be ready," Lt. Gen. Jeffrey Harrigian told reporters on Tuesday (13 September) at the Pentagon.

Road to Mosul
A tank near the recaptured airbase at Qayyarah. REUTERS/Stringer

"This is a partnered effort. This is something we're working from both the land component perspective with the Iraqis and clearly ensuring that, as we begin to put some of our airplanes in there in the future, that it's got the capabilities that we need," Harrigian added, describing efforts to prepare Qayyarah.

The 400 US troops at the airbase are among the 560 soldiers President Barack Obama approved for deployment in Iraq in July. According to the Pentagon, there are 3,800 members of the US armed forces in Iraq.

Mosul is the second-largest city in Iraq and presents a significant challenge to coalition forces.

"Operations against Mosul have already started… with a population of around 665,000 people, the operation against [Isis] will be much more complicated than the successful operation against Isil in Ramadi late last year," Marine Corps Gen. Joseph F. Dunford Jr. said.

Isil – Islamic State of Iraq and the Levant – is an abbreviation of Islamic State, which is used by the US government.

The next stage of the campaign will involve attempts to seize towns and villages on the southern outskirts of Mosul.

Iraqi security forces airdropped thousands of leaflets south of the city over the weekend warning citizens in anticipation of a new offensive.

"Protect yourself, don't be human shields for the enemy, leave the town immediately," a leaflet seen by CNN read.