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One of Barack Obama's former rivals for the US presidency has said a vote against taking military action in Syria would be "catastrophic".
Republican Senator John McCain, an advocate of military intervention following an attack in Damascus last month, met the President for talks in the White House.
It came as Mr Obama began attempts to convince a sceptical Congress of the need to intervene.
Members are due to return from their summer break next week to discuss the crisis and decide whether to support strikes against Bashar al Assad's regime, which the US holds responsible for the attack.
Meanwhile, a spokesman for the Obama administration said the White House was prepared to rework language in a draft resolution authorising military force in Syria.
Mr McCain, who previously said the threat of a limited strike does not go far enough, spoke of an attack that would "degrade Assad's capabilities" and allow opposition fighters to "reverse momentum on the battlefield".
He said he was encouraged by the meeting but warned there was "a long way to go" before the resolution is passed, with some Republican and Democrat lawmakers against any form of military action.
The US has accused the Assad regime of orchestrating the attack in Damascus last month, which it claims killed more than 1,400 civilians, including 400 children.
Secretary of State John Kerry said new physical evidence in the form of blood and hair samples pointed to the use of sarin gas in the August 21 attack.
"We know the regime ordered this attack," he said.
"We know they prepared for it. We know where the rockets came from. We know where they landed. We know the damage that was done afterwards."
As part of their appeal for support, administration officials have briefed lawmakers in private to explain why the US feels compelled to act against Mr Assad.
Top officials including Mr Obama and Vice President Joe Biden made calls to individual lawmakers.
Meanwhile, the USS Nimitz aircraft carrier and four other Navy vessels sailed into the Red Sea, in a move described by retired Admiral Gary Roughead, chief of naval operations during the 2011 strikes on Libya, as "prudent planning".
The US already has five destroyers and an amphibious ship carrying hundreds of Marines in the eastern Mediterranean.