"Investigation proceedings have started with 1,387 members of the Daesh terrorist organisation who were recently transferred from the Syrian territory," the judiciary's media office said in a statement, using the Arabic acronym for IS.

"Under the supervision of the head of Iraq's Supreme Judicial Council, several judges specialising in counterterrorism started the investigation."

Those detainees are among 7,000 IS suspects, previously held by Syrian Kurdish fighters, whom the US military said it would transfer to Iraq after Syrian government forces recaptured Kurdish-held territory.

They include Syrians, Iraqis and Europeans, among other nationalities, according to several Iraqi security sources.

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.

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 said its investigation procedures "will comply with national laws and international standards".