The U.S. intelligence community believes Syria used chemical weapons on anti-government rebels.
The White House on Thursday informed Congress that intelligence shows that Syrian President Bashar Assad ordered government troops to use sarin gas against the rebels.
"Our intelligence community does asses with varying degrees of confidence that the Syrian regime has used chemical weapons on a small scale in Syria, specifically the chemical agent sarin," Miguel Rodriguez, director of the White House office of legislative affairs, wrote in a letter.
Rodriguez said the use of chemical weapons in Syria is a "red line" for the United States. But he said that the U.S. is not ready to take action yet.
"Given the stakes involved, and what we have learned from our own recent experience, intelligence assessments alone are not sufficient -- only credible and corroborated facts that provide us with some degree of certainty will guide our decision-making, and strengthen our leadership of the international community," Rodriguez wrote.
Also Thursday, U.S. Defense Secretary Chuck Hagel, speaking in Abu Dhabi, reiterated the message in the letter, saying that the use of chemical weapons "violates every convention of warfare."
Hagel said Wednesday that an Israeli official's revelation that Israel believes Syria used chemical weapons caught him by surprise.