The head of the US military's Task Force Iraq, Brigadier General William Seely, sent a letter to the head of Iraq's joint operations command, a copy of which was seen by AFP.

The letter said forces from the US-led coalition in Iraq would "be repositioning forces over the course of the coming days and weeks to prepare for onward movement".

"In order to conduct this task, Coalition Forces are required to take certain measures to ensure that the movement out of Iraq is conducted in a safe and efficient manner," said the letter, dated Monday.

As the letter was signed by a US official, it was not immediately clear whether it applied to forces from the 76 countries which make up the international coalition.

A US defence official and an Iraqi defence official confirmed the letter was real and had been delivered.

It said helicopters would be travelling in and around the Green Zone as part of the preparations.

AFP could hear helicopters flying low over Baghdad throughout the night on Monday.

Some 5,200 US soldiers are stationed across Iraqi bases to support local troops preventing a resurgence of the Islamic State group.

They make up the bulk of the broader coalition, invited by the Iraqi government in 2014 to help combat the jihadists.

On Sunday, Iraq's parliament voted in favour of rescinding that invitation and ousting all foreign troops.

It came in reaction to a US precision drone strike on Baghdad that killed Iranian general Qasem Soleimani and top Iraqi commander Abu Mahdi al-Muhandis, among others.

On Monday, Iraqi premier Adel Abdel Mahdi met with the US Ambassador Matthew Tueller, telling him it was "necessary to work together to withdraw foreign forces from Iraq".