White House: More evidence needed on Syrian chemical weapon claim

While there is evidence that Bashar al-Assad's Syrian regime used chemical weapons on its own people, clearer evidence and facts are required before acting, a White House official said on Thursday.

The official spoke to reporters on Thursday after the White House released a letter to Senate leaders saying that the U.S. intelligence community can assess with varying degrees of confidence that Assad's regime has used chemical weapons on a small scale. The letter specifically mentioned the chemical agent sarin.

The official said more facts must be obtained before the U.S. can act on the reports.

"The chain of custody is not clear, so we cannot confirm how the exposure occurred and under what conditions," the official said. "If we reach a definitive determination that this red line has been crossed based on credible corroborated information, what we will be doing is consulting closely with our friends and allies and the international community more broadly, and the Syrian opposition, to determine what the best course of action is. All options are on the table, and it could run... a broad spectrum of activity across... various lines of activity in Syria."

President Barack Obama called on the U.N. to lead a comprehensive evaluation of the evidence and determine exactly what took place. The U.S. will work with allies, friends and the Syrian opposition during the investigation to obtain and evaluate additional information.

"Given the stakes involved, given how serious the situation is and what we've learned from our own recent experience, intelligence assessments are not alone sufficient (for action)," the official said. "Only credible and corroborated facts that provide us with some degree of certainty can then guide our decision making."

News reports have said Syrian forces allegedly used chemical weapons in Homs, Damascus and Aleppo.