The detainees had been held in Syria for years in camps overseen by Kurdish-led forces, but the recapture of surrounding territory by Damascus prompted Washington to step in.

CENTCOM said it "completed a transfer mission following a nighttime flight from northeastern Syria to Iraq on Feb 12 to help ensure ISIS detainees remain secure in detention facilities."

"The 23-day transfer mission began on Jan 21 and resulted in US forces successfully transporting more than 5,700 adult male ISIS fighters from detention facilities in Syria to Iraqi custody," it added in a statement.

The US had previously announced it would transfer around 7,000 detainees

"Job well done to the entire Joint Force team who executed this exceptionally challenging mission on the ground and in the air with great focus, professionalism, and collaboration with our regional partners," said Admiral Brad Cooper.

"We appreciate Iraq's leadership and recognition that transferring the detainees is essential to regional security," he added.

The vast majority of the detainees are Syrians, while they also include hundreds of foreigners from Arab countries as well as Europe, Asia and Australia, a senior Iraqi security source told AFP earlier this week.

Many prisons in Iraq are already packed with IS suspects, and courts have handed down hundreds of death sentences and life terms to those convicted of terrorism offences, including foreign fighters.

Earlier this month, Iraq's judiciary said it had begun investigations into detainees transferred from Syria.

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.

Last 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.