The detainees from some 60 countries had for years been held in Syrian prisons run by Kurdish-led forces before 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.

IS swept across Syria and Iraq in 2014, 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.

- 61 countries -

Last month, Syrian troops drove Kurdish forces from swathes of northern Syria, sparking questions over the fate of the IS prisoners.

Lingering doubts about security pushed Washington to announce it would transfer them to Iraq to prevent "a breakout" that could threaten the region.

"We appreciate Iraq's leadership and recognition that transferring the detainees is essential to regional security," said head of CENTCOM Admiral Brad Cooper.

"Job well done to the entire Joint Force team who executed this exceptionally challenging mission on the ground and in the air," he added.

Iraq's National Centre for International Judicial Cooperation (NCIJC) said 5,704 IS detainees of 61 nationalities have arrived in Iraq.

They include 3,543 Syrians, 467 Iraqis, and another 710 detainees from other Arab countries.

There are also more than 980 foreigners including those from Europe, Asia, Australia and the United States.

The NCIJC said Iraq's judiciary will interrogate the detainees before taking legal action against them.

Many prisons in Iraq are already packed with IS suspects.

Iraqi courts have handed down hundreds of death sentences and life terms to those convicted of terrorism offences, including foreign fighters.

Under Iraqi law, terrorism and murder offences are punishable by death, and execution decrees must be signed by the president.

The detainees in Syria were transferred to Baghdad's al-Karkh prison, once a US Army detention centre known as Camp Cropper, where former ruler Saddam Hussein was held before his execution.

To make space for the newcomers, authorities moved thousands of prisoners from the Karkh prison to other facilities, a lawyer and an inmate told AFP on condition of anonymity.

- Repatriation -

Iraq has issued calls for countries to repatriate their nationals among the IS detainees, though this appears unlikely.

For years, Syria's Kurdish forces also called on foreign governments to take back their citizens, but this was done on a small scale limited to women and children held in detention camps.

Most foreign families have left northeast Syria's Al-Hol camp, which holds relatives of IS fighters, since the departure of Kurdish forces who previously guarded it, humanitarian sources told AFP on Thursday.

Last month, the Syrian government took over the camp from Kurdish forces who ceded territory as Damascus extended its control across swathes of Syria's northeast.