Defense Secretary Chuck Hagel declined Wednesday to discuss Syria's possible use of chemical weapons against rebels, raising doubts over whether Washington still viewed chemical arms a "red line."

The United States has avoided intervention in the Syrian conflict but President Barack Obama has repeatedly warned Damascus against resorting to chemical weapons, evoking the possibility of US military action.

When asked direct questions by senators as to whether Syria fired chemical weapons, both Hagel and General Martin Dempsey, chairman of the Joint Chiefs of Staff, sidestepped the sensitive issue.

They told lawmakers the US intelligence chief, James Clapper, would likely address their queries behind closed doors Thursday.

"Our intelligence agencies are going into more detail on what we know and what we don't know," Hagel told the Senate Armed Services Committee.

"When General Clapper is before you tomorrow, he will get into that, but I suspect that some of this will have to be done in closed session," Hagel said when asked by Senator Carl Levin.

Dempsey also refused to answer a yes-or-no question from Republican Senator John McCain on whether the Syrian regime had employed weapons from its chemical arsenal.

"I think director Clapper, he may have to take you into closed session to answer that question," Dempsey said.

He said the United States was "eager" for a UN team to investigate reports about the possible use of chemical weapons and added that he could not say more in public.

Obama said last year that if the regime tried to move or use chemical weapons, that would "change my calculus" and that President Bashar al-Assad would be held accountable by the international community.

Fresh questions about US policy on Syria came as Hagel announced the Pentagon was reinforcing a US contingent in Jordan that has been deployed to help secure chemical weapons if necessary and prepare for a possible spillover of violence.

Last year, about 150 US military specialists were sent to Jordan and Hagel said he had ordered a US Army headquarters team to bolster the mission, bringing the total American presence to more than 200 troops, officials said.

"These personnel will continue to work alongside Jordanian Armed Forces to improve readiness and prepare for a number of scenarios," Hagel said.