US President Barack Obama has warned Syria that its use of chemical weapons would be a "game changer" for the United States.
However, he made it clear he was in no rush to intervene in the civil war there on the basis of evidence he said was still preliminary
Speaking a day after the disclosure of US intelligence that Syria had likely used chemical weapons against its own people, Mr Obama talked tough while calling for patience as he sought to fend off pressure for a swift response against Syrian President Bashar al-Assad.
"Horrific as it is when mortars are being fired on civilians and people are being indiscriminately killed, to use potential weapons of mass destruction on civilian populations crosses another line with respect to international norms and international law," Mr Obama told reporters at the White House as he began talks with Jordan's King Abdullah.
"That is going to be a game changer," he said.
But Obama stopped short of declaring that Mr Assad had crossed "a red line" and described the US intelligence evaluations as "a preliminary assessment."
While some more hawkish lawmakers have called for a US military response and for the arming of anti-Assad rebels, several leading congressional voices urged a calmer approach after Secretary of State John Kerry briefed them.
"This is not Libya," said Nancy Pelosi, the senior Democrat in the House of Representatives, referring to the relative ease with which a NATO bombing campaign helped overthrow Muammar Gaddafi in 2011.
"The Syrians have anti-aircraft capability that makes going in there much more challenging."
US officials said on Thursday the intelligence community believes with varying degrees of confidence that Assad's forces used the nerve agent sarin on a small scale against rebel fighters.
Mr Obama had warned earlier that deployment of chemical weapons by the Syrian government would trigger unspecified consequences, widely interpreted to include possible US military action.
Aides have insisted that the Democratic president will need all the facts before he deciding on action, making clear it is mindful of the lessons of the start of the Iraq war more than a decade ago.
Then, the Republican administration of President George W Bush used inaccurate intelligence to justify the invasion of Iraq in pursuit of nuclear, chemical and biological weapons that turned out not to exist.
Syria denies using chemical weapons in the two-year-old conflict in which more than 70,000 people have been killed.