Top Republican warned Monday of catastrophic consequences if Congress votes against striking Syria and suggested the White House may be mulling a more robust military intervention.

Top Republican warned Monday of catastrophic consequences if Congress votes against striking Syria and suggested the White House may be mulling a more robust military intervention.

Senators John McCain, a Republican hawk who have long advocated deeper US involvement in Syria's civil war, met President Barack Obama as part of his efforts to sell US lawmakers on military action.

McCain emerged from the meeting with a stern warning for fellow Republicans who may be considering voting against military action purely to damage the Democratic president.

"A vote against that resolution by Congress I think would be catastrophic," said McCain.

"It would undermine the credibility of the United States of America and the president of the United States. None of us want that."

McCain suggested that more US muscle than expected might be deployed.

"We have been given some reason to believe that very serious strikes may take place as opposed to cosmetic," McCain said.

"We now need to see a lot of the details."

The plan could be intended to degrade President Bashar al-Assad's capabilities and upgrade those of militant groups, McCain suggested.