The US Senate debated a defense bill Tuesday that would ease restrictions on detainee transfers from Guantanamo, a major first step toward closing the controversial war-on-terror prison.

Senate Majority Leader Harry Reid said the provision should be the first measure debated and voted on in the defense spending bill for 2014 now under consideration, and which Congress is aiming to pass in December.

President Barack Obama's Democrats and rival Republicans are largely split on the future of the detention facility at the US Naval base at Guantanamo Bay, Cuba.

Shortly after his 2009 inauguration Obama pledged to close the prison, but the goal has eluded him, complicated by claims from opponents that some released detainees went on to join terror groups back home.

The current measure would loosen restrictions on moving detainees out of the prison, including those who have long been cleared for transfer overseas but are still held.

It would also lift a ban on sending terror suspects from Guantanamo to the US mainland for detention, trial or emergency medical treatment.

"I would accept the language in that defense bill as it relates to Guantanamo as a significant improvement. But my Republican colleagues want to have an opportunity to change that," Reid said on the Senate floor.

The issue, he added, would "need to be resolved on the Senate floor."

Guantanamo still holds 164 "war on terror" suspects, including 56 Yemenis who were deemed not to be a security threat but were under a moratorium until May for transfer to their violence-wracked country.

Most have never been charged or tried and face indefinite detention for so far unproven suspicions.

On Monday the White House issued a statement calling the Armed Services Committee's Guantanamo proposal "constructive" and said it wanted to work with Congress to ensure "flexibility" in allowing the administration to transfer such detainees if security threats were sufficiently mitigated.

Senator James Inhofe, the top Republican on Armed Services, said "sticking points" remained.

"I feel very strongly about that issue," Inhofe told AFP, referring to his opposition to the easing of Guantanamo restrictions.

"I like the language in the House bill on Gitmo."

The Republican-led House of Representatives' defense spending bill, which passed last June, would bar the administration from transferring detainees to US soil or a foreign country such as Yemen.

Armed Services chairman Senator Carl Levin on Tuesday acknowledged the tough task of forging a compromise, saying that finishing the mammoth legislation "is going to be a very difficult task."

"We are going to need the cooperation of all senators to get this important bill passed, as we must, in the limited time available to us before Thanksgiving week."

The Senate and House would then appoint negotiators to thrash out a spending bill by year end.

But Senator Jeff Sessions accused Reid of "overreaching" by steamrolling ahead with just a handful of opportunities to introduce amendments, and suggested he and fellow Republicans would obstruct efforts to ram through a bill without sufficient debate.