WASHINGTON (AFP) - The White House revealed to lawmakers on Tuesday details of its request to Congress for a three-year war authority to battle Islamic extremists that would prohibit "enduring" offensive combat operations.

The authorisation for use of military force (AUMF) would also allow for use of US special forces, and would not restrict operations to a geographic area, several Democratic senators said after a closed-door briefing by White House officials.

President Barack Obama's team has been deliberating for months on how to move ahead in the next phase of its years-long war against militant extremists.

The White House is expected this week to unveil its proposal for authority to combat the Islamic State in Iraq and Syria extremist group, and administration officials have stepped up their deliberations with congressional leaders.

The new AUMF would supersede the open-ended war authority which Congress passed in the aftermath of the 9/11 attacks in 2001.

The administration currently uses the 2001 authority to prosecute military action against ISIS, in particular US-led air strikes on the group's forces in Iraq and Syria.

Lawmakers have stressed that no official war authority language has been released.

"It was an opening salvo," Senate Democratic leader Harry Reid said after the briefing. "There was nothing finalised yet."

Democratic Senator Robert Menendez said a key plank of the proposal, according to the White House briefers, required "no enduring offensive combat" operations, a qualifier he acknowledged could face opposition in Congress.

"Unless that is further defined, that might be seen as too big a statement to ultimately embrace because, forget about Barack Obama, there would be a new president in two years," Menendez said.

"The challenge to get a broad bipartisan AUMF is threading the needle between not having too expansive an authorisation that can be used for an open-ended, prolonged conflict, and however giving an authorisation that gives the President the wherewithal to effectively defeat ISIL." ISIL is another acronym for ISIS.

Senator Dianne Feinstein said she understood that the request for a three-year authorisation would allow for special operations.

But "I think the big discussion will be over how you word the language on troops," the top Democrat on the Senate Intelligence Committee said.

Some Republicans were already pushing back.

"I will agree to nothing that restricts the President's latitude of action," hawkish Senator John McCain told reporters.

Language barring enduring combat operations "would be unacceptable to me," he said, adding that any debate about the war authority should not be rushed.

Republican Senator Lindsey Graham said he was briefed by the White House by phone and was disturbed that the new authority would not allow US forces to protect US-trained Syrian rebels against air attacks by strongman Bashar al-Assad's military.

"I'll be a no to that," he said.