Members of Congress urged US President Barack Obama to take action to "secure" Syria's chemical weapons after he warned Thursday that strongman Bashar al-Assad likely used them against his own people.

Republican Senator John McCain led the revulsion and anger in Congress, saying it was now up to Obama to coordinate a response that prevents such weapons, including the agent sarin, from falling into the hands of terrorists or extremist groups.

Obama "said that if Bashar Assad used chemical weapons, it would be a game-changer, that it would cross a red line. I think it's pretty obvious that a red line has been crossed," McCain told reporters.

"We have to have operational capability to secure these chemical weapon stocks," he added. "We do not want them to fall into the wrong hands, and the wrong hands are a number of participants in the struggle that's taking place in Syria."

For months the veteran Republican has urged Obama to take a more pro-active role in the Syrian conflict and pressed him to help arm Syrian rebels and ensure safe havens in the country.

On Thursday he called for increased White House pressure on Russia and Iran to stop supplying weapons to Assad, and greater commitment to aid Syria's rebel groups fighting Damascus.

Democrat Dianne Feinstein, chair of the Senate Intelligence Committee, concurred that "red lines have been crossed" in Syria.

"Action must be taken to prevent larger scale use. Syria has the ability to kill tens of thousands with its chemical weapons," she said in a statement.

"The world must come together to prevent this by unified action which results in the secure containment of Syria's significant stockpile of chemical weapons."

But while Feinstein turned toward a United Nations role, some Republicans were urging more direct US action.

"The time for passive engagement in this conflict must come to an end," Senator Marco Rubio said, adding that allowing Assad to plunge his country deeper into chaos "will have disastrous consequences for US interests for decades to come."

"I urge President Obama to explain to Congress and the American people... what additional measures he is ready to take to follow through on his previous statements."

Republican Howard "Buck" McKeon, chairman of the House Armed Services Committee, said Assad's use of chemical weapons on the Syrian people, if true, is "an astounding violation of human rights," and triggers a "national security imperative."

Obama now has "a deep moral imperative" to act, McKeon said.

House Intelligence Committee chairman Mike Rogers said Obama made an "important acknowledgement" about chemical weapons.

"Now that we have confirmed their use, the question is what is our plan for transition to a post-Assad Syria?" he said.

"The world is waiting for American leadership."