Washington (AFP) - US lawmakers could vote within days to authorize the military to arm and train moderate Syrian rebels, with Democrats drafting the legislation Wednesday in an effort to reverse advances by the Islamic State (IS).

The possible move follows a last-minute request by President Barack Obama, who was scheduled to address the nation later Wednesday and lay out his plan to defeat the jihadist group that has rampaged across parts of Syria and Iraq.

With the White House pushing Congress to include the rebel-training authority in a temporary government spending bill, the Republican-led House abruptly suspended that measure, a so-called continuing resolution (CR) which had been unveiled the previous night and was set for a Thursday vote.

"Given the severity of the situation and the need for all members to properly evaluate the president's request, the House will postpone consideration of the continuing resolution," House Majority Leader Kevin McCarthy announced.

Democratic leaders including Senate Majority Leader Harry Reid are strongly backing Obama, who appeared poised to green-light air strikes against IS in Syria.

Coordinating with rebels on the ground would be key to that strategy.

"It's clear to me that we need to train and equip Syrian rebels and other groups in the Middle East that need some help," Reid told colleagues.

"The president has tried to get that (authority) from us, and we should give it to him," Reid said.

In June the White House asked lawmakers for $500 million to train and equip vetted Syrian rebels, in what would be a significant escalation of US involvement in the conflict, following months of pressure from some lawmakers to ramp up support.

Reid noted that some Republicans remain concerned about costs, particularly before Obama lays out his strategy, but he stressed "there is money to do" the training.

Reid and other congressional leaders including House Speaker John Boehner met Tuesday with Obama to discuss his strategy.

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The White House released details of Obama's Wednesday call with Saudi King Abdullah in which "the president and the king agreed on the need for increased training and equipping of the moderate Syrian opposition."

That authority could come in the form of a stand-alone measure, but momentum appeared to be building for including it in the CR.

The measure would fund the government at current levels until December and must be passed before fiscal year 2015 begins on October 1.

With members of Congress heading home as early as September 19 and not returning until after the congressional mid-term elections on November 4, Democrats see the CR as the safest vehicle for the authorization.

"I would hope that it would be on there," top House Democrat Nancy Pelosi told reporters.

House Appropriations Committee chairman Hal Rogers pushed back.

"I hate to see this attached to a continued resolution at the very last minute," Rogers said.

"This is a complex, complicated policy change that needs to be fully vetted here in the Congress."