The move comes after President Donald Trump in January signed an executive order reversing his predecessor Barack Obama's ultimately fruitless 2009 directive to shutter the facility that has drawn global scorn.

Mattis's recommendations provide "our warfighters guidance on nominating detainees for transfer to Guantanamo detention should that person present a continuing, significant threat to the security of the United States," Pentagon spokeswoman Commander Sarah Higgins said.

The Pentagon did not provide any additional details on the recommendations, and it was not clear what impact they would have.

The president's National Security Council declined comment.

Guantanamo has not received any new inmates since 2008, but on the campaign trail Trump vowed to load the facility with "bad dudes," and said it would be "fine" if US terror suspects were sent there for trial.

Any attempt to send new inmates to Guantanamo would likely run into a plethora of legal challenges.

"Given the history of torture, unlawful detention and complete lack of justice provided there, no new detainees should ever be transferred to Guantanamo," Daphne Eviatar, a director at Amnesty International USA, said in a statement.

Trump's January executive order gave Mattis and the heads of other agencies 90 days to recommend policies on wartime detainees and whether they should be sent to Guantanamo.

US military officials have been openly discussing the fate of Islamic State group detainees, mainly foreign fighters, held by US-backed militias in northern Syria.

Mattis on Monday said the US-backed Syrian Democratic Forces, who have been fighting IS, had more than 400 prisoners.

The Pentagon chief added that he was "absolutely certain that there is not one thing going on down there (at Guantanamo) that would not be in accordance with" the Geneva Conventions on the treatment of prisoners of war.