"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 Friday.

The transfer is expected to take several days.