The top US Republican said Tuesday he backed President Barack Obama's "sound" plan to arm and train vetted Syrian rebels for battling extremists and called on Congress to authorize the action.

"Frankly I think the president's request is a sound one," House Speaker John Boehner told reporters after convening a caucus meeting to convince members to approve the issue in a crucial vote expected Wednesday.

"There's no reason for us not to do what the president asked us to do," he added. But he reiterated a common theme emerging among Republican conservatives that there was "a lot more" Washington could do to battle the Islamic State (IS) group rampaging across parts of Iraq and Syria.

"If our goal here is to destroy ISIL," Boehner said, using another name for the group, "we've got to do more than train a few folks in Syria, and train a few folks in Iraq, and dropping bombs."

Lawmakers said they expect Congress will approve the measure this week, in the form of an amendment to a stop-gap government funding bill, before recessing until after the November 4 congressional elections.

Boehner and others urged that a broader debate over American use of force against IS jihadists be launched after election day.

Congressman Tom Cole said conservatives worry the authorization for training the Free Syrian Army was too narrow, but added he believed there was sufficient support to pass it.

"We need that larger debate, but this is what the president's asked for right now," Cole said.

"We're certainly going to give him what he asked for, I think, or at least a majority of us will."

House Armed Services Committee chairman Buck McKeon, who introduced the measure, tailored it in a way that puts checks on Obama's powers.

It requires the administration to keep Congress in the loop with reports to lawmakers every 90 days, authorizes action only through mid-December and prohibits Obama from dispatching US combat troops.

Nevertheless, McKeon said he feels Obama should be doing more to address the threat.

"We need to galvanize and go full bore after ISIL," he said.

Lawmakers also appear to be in broad agreement that Obama will ultimately need new authorization for the use of military force against IS, rather than depend on authority put in place following the attacks of September 11, 2001.

"Most members feel, like me, that going into military action on an authorization of 2001 and 2002 in a different place against a different enemy -- that was never meant to be," Cole said.

The amendment was to receive six hours of debate on the House floor Tuesday.