New emphasis on assisting opposition forces, time limits and restrictions on use of US ground troops wins Republicans over
The prospect of air strikes in Syria has grown with political developments driving towards military action.
President Barack Obama secured the support of Republican leaders in Congress and key senators agreed to back legislation ruling out the use of United States ground troops in any military response to a suspected chemical weapons attack.
Obama promised that strikes would be part of a "broader strategy" to support rebel forces.
Having previously insisted a US attack would punish Assad for using chemical weapons but not aim at regime change, Obama stressed for the first time that it would "degrade Assad's capabilities" and help secure a "transition" by ending the two-and-a-half-year conflict.
"We have a broader strategy that will allow us to upgrade the capabilities of the opposition, allow Syria ultimately to free itself from the kinds of terrible civil war, death and activity that we've been seeing on the ground," said Obama, as he met congressional leaders at the White House.
His new emphasis promptly secured the support of John Boehner, the Republican House Speaker, whose endorsement could be vital to the White House's efforts to convince a sceptical Congress to endorse US military action next week. "I'm going to support the President's call for action," Boehner said.
Obama also tried to assure the public that involvement in Syria will be a "limited, proportional step".
White House aides were increasingly optimistic that Obama would get the go-ahead for intervention from Congress next week, after his former rival Senator John McCain offered to help him avoid a humiliating defeat in exchange for assurances the US would do more to back the rebels.
"An authorisation for some limited use of force is all but certain to pass on a bipartisan basis," said a senior Republican Senate aide.
Legislators in both the Republican and Democratic parties had called for changes in the President's requested legislation, insisting it be rewritten to restrict the type and duration of any military action.
Obama signalled he was open to that. "This is not Iraq and this is not Afghanistan," he said.
Officials said the emerging Senate measure was expected to be approved today by vote in the Senate Foreign Relations Committee.
In the Senate, the compromise was the work of Senators Bob Menendez, the Democratic chairman of the Senate Foreign Relations Committee, and Bob Corker, the senior Republican on the panel.
The proposed measure would set a time limit of 60 days for any military action and says the President could extend that for 30 days more unless Congress has a vote of disapproval. The measure also bars the use of US ground troops for "combat operations".
Earlier, Secretary of State John Kerry, testifying before the committee, signalled that the troop restriction was acceptable to the Administration. "There's no problem in our having the language that has zero capacity for American troops on the ground," he said.
Kerry had said he would prefer not to have such language, hypothesising the potential need for sending ground troops "in the event Syria imploded" or to prevent its chemical weapons cache from falling into the hands of a terrorist organisation.
But he added: "President Obama is not asking America to go to war. This is not the time to be spectators to slaughter."
In other developments:
Downing Street indicated David Cameron could yet return to the House of Commons with a proposal for Britain to arm the rebel forces;
Israel heightened tensions in the region by carrying out a test of a new missile-defence system, launching a missile from the Mediterranean Sea with US support;
A Syrian forensic medicine expert claiming to have evidence that Bashar al-Assad's forces used chemical weapons in an attack near the city of Aleppo in March defected to Turkey.
Ban Ki Moon, the UN Secretary-General, warned military action might only unleash more turmoil.
"We must consider the impact of any punitive measure on efforts to prevent further bloodshed and facilitate the political resolution of the conflict. I call for the Security Council to unite should the allegations prove true."
Obama is in Sweden today and will fly on to Russia for the G20 summit, leaving his senior aides and Vice-President Joe Biden to continue lobbying members of Congress still reluctant to give their support.
Several congressmen stressed that the US public seemed overwhelmingly to oppose military action and said they were coming under pressure to block Obama's plan.
Arkansas senator Tim Griffin said that of 225 emails he received, only three supported US military action.