The White House battled Monday for congressional authorization to bomb Syria, as two top Republicans warned that a "no" vote after President Barack Obama had threatened action would be catastrophic.

Hawkish senators John McCain and Lindsey Graham emerged from an hour of talks with Obama suggesting that the White House could be mulling a wider military campaign in Syria than first thought, along with more robust support for the opposition.

Obama shocked Washington and the world on Saturday when he decided to seek support for military action in Syria from Congress, when it seemed US cruise missile strikes on President Bashar al-Assad's forces and assets were imminent.

McCain and Graham appeared to offer qualified backing for Obama's plans.

"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."

Graham warned of the wider consequences of a failure to back military action.

"I can't sell another Iraq or Afghanistan, because I don't want to," Graham said.

"(What) I can sell ... (is) that if we don't get Syria right, Iran is surely going to take the signals that we don't care about the nuclear program. "

The senators also hinted at the administration's evolving strategy for Syria.

Obama has stressed that any US action, expected to include cruise missile attacks, would be "limited" and "narrow."

But McCain said he had "been given some reason to believe that very serious strikes may take place as opposed to cosmetic (ones)."

"I don't think it is an accident that the aircraft carrier is being moved over in the region."

ABC News reported Monday that the USS Nimitz aircraft carrier was moving westward toward the Red Sea, though had not yet received orders to support a strike on Syria.

Obama's plan could be to degrade President Bashar al-Assad's capabilities and upgrade those of vetted opposition groups, McCain suggested.

Graham indicated that the administration, which resisted arming Syrian rebels for months, may be prepared to stiffen a nascent plan -- announced after previous, small scale chemical weapons attacks -- to increase military aid to some rebel groups.

"There seems to be emerging from this administration a pretty solid plan to upgrade the opposition," Graham said, saying regional players like Saudi Arabia, Turkey and Jordan should play a key role.

Though McCain and Graham's comments would have pleased the White House, they have long supported military action against Syria so are an easier sell than many other Republicans.

Secretary of State John Kerry and Defense Secretary Chuck Hagel will testify to the Senate Foreign Relations committee Tuesday in what will be one of the most high profile political set pieces in Washington in weeks.

America's top military officer, General Martin Dempsey, chairman of the joint chiefs of staff, will also appear.

The administration may have its work cut out for it in the Republican-run House, which includes many conservatives who have blocked Obama's agenda at home on issues like gun control and immigration reform and may relish the chance to embarrass him abroad.

Liberal House Democrats wary of seeing their country sucked into another prolonged engagement in the Middle East are also a cause for White House concern.

With that in mind, top administration officials including Kerry and Hagel held an unclassified briefing for House Democrats on Monday, while most of official Washington was on hiatus for the Labor Day holiday.

A senior White House official said that Kerry will also testify on Wednesday before the House Foreign Affairs committee. Classified briefings will meanwhile be offered to lawmakers throughout the week.

Obama has a limited window this week for personal politicking on Syria: he is due to leave town on Tuesday evening to visit Sweden and attend the G20 summit in Russia.

The president made calls over the weekend to individual members of Congress, as did Vice President Joe Biden.

A senior White House official said the administration would deploy all of its resources on Capitol Hill and beyond to sway opinion on the Syria vote.

On Tuesday, before leaving for Europe, Obama will meet the chairs and top opposition members of key national security committees from both the Senate and the House.