The United States will "shortly" begin arming Syrian rebels, looking to boost moderate factions over al-Qaida-affiliated extremists whose rise would be a national security "nightmare," the top Republican on the Senate Foreign Relations Committee told CBS News on Tuesday.

"I do think we’ll be arming the opposition shortly," Sen. Bob Corker of Tennessee said in an interview. "We’re doing a lot more there on the ground than really is known, but we do have to change the equation."

"I think you all know the moderate opposition groups that we support are not as good at fighting, they’re not as good as delivering humanitarian aid, and we need to change the balance," Corker said.

President Barack Obama has resisted giving high-tech weapons to rebels fighting to topple strongman Bashar Assad in a civil war that has claimed at least 70,000 lives, according to United Nations estimates. But there have been news reports over the past year that the United States has agents on the ground helping to steer weapons to rebel groups thought to be more moderate. And recent U.S. intelligence disclosures that Assad's forces may have used chemical weapons have ramped up pressure on Obama to do even more. The president has long seemed disinclined to escalate the American role.

Corker suggested that arming friendly rebel groups could prompt Russia, a longtime patron of Assad, to embrace calls for a political agreement that sees the Syrian president relinquish power. And he expressed hope that it would boost moderates over more effective rebel fighting forces like the al-Qaida-affiliated al-Nusra Front.

"A nightmare would be al-Nusra, if you will, gaining control of Syria," Corker said. "That’s worse than Assad being there."

And the senator suggested that the United States could keep any weapons it provides out of the hands of extremist fighters.

"I think we can. We’ve known for a long time which of the groups are more moderate and more secular," he said, before acknowledging that "you never have total control."

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Corker's comments came a day after Senate Foreign Relations Committee Chairman Bob Menendez, a Democrat, introduced legislation to arm Syrian rebels, punish countries that arm Assad's regime, and stabilize the country when and if he loses power.

Menendez's measure would greenlight the flow of arms from the United States to rebel groups "that have gone through a thorough vetting process" to assess their commitment to human rights and curbing the spread of weapons know-how. It would forbid the transfer of shoulder-fired surface-to-air missiles. The legislation would also create a $250 million "transition fund" and enable Obama to impose sanctions on any entity that provides Assad's regime with military equipment or oil.

National Security Council spokeswoman Caitlin Hayden declined to comment specifically on Menendez’s proposal, but stressed that “we are working urgently to end the conflict in Syria and hasten a transition from Bashar al-Assad to a democratic Syria that is inclusive of all Syrians.” She also noted that Obama has imposed sanctions on key regime figures and supporters five separate times since March 2011.

“As the president has said, we continue to explore every available, practical and responsible means to end the suffering of the Syrian people and accelerate a political transition,” Hayden told Yahoo News.