"The prisoner transfer operation is ongoing, with US forces transporting detainees by land and air," an Iraqi security official said, adding that "up to 1,000 IS detainees are expected to arrive in Iraq today".

Another security source confirmed the transfer was underway, saying the detainees -- who include Iraqis and Europeans -- will be distributed among at least three prisons in Iraq.

The group is the second batch of 7,000 IS suspects, previously held by Syrian Kurdish fighters, that the US military said it would transfer to Iraq after Syrian government forces recaptured Kurdish-held territory.

Europeans were also among the 150 senior IS detainees who were the first to be transferred on Wednesday, two Iraqi security officials told AFP on Friday.

The transfer is expected to take several days.

In 2014, IS swept across Syria and Iraq, committing massacres and forcing women and girls into sexual slavery.

Backed by US-led forces, Iraq proclaimed the defeat of IS in the country in 2017, and the Kurdish-led Syrian Democratic Forces (SDF) ultimately beat back the group in Syria two years later.

The SDF went on to jail thousands of suspected jihadists and detain tens of thousands of their relatives in camps.

This month, the United States said the purpose of its alliance with Kurdish forces in Syria had largely expired, as Damascus pressed an offensive to take back territory long held by the SDF.

In Iraq, where many prisons are packed with IS suspects, courts have handed down hundreds of death sentences and life terms to people convicted of terrorism offences, including many foreign fighters.

Iraq's judiciary has said it will launch legal proceedings against the detainees brought from Syria.

The first Iraqi security official said the country had formed a committee comprising the justice ministry, air forces and anti-terrorism forces to coordinate the transfers from Syria as part of the US operation.

Amnesty International has called on the US to "urgently put in place safeguards before making any further transfers", and urged Iraq to hold "fair trials, without recourse to the death penalty".