Defense Secretary Chuck Hagel said Thursday that the Syrian regime had used chemical weapons against its own people, an action that crosses President Obama’s so-called red line for intervening in the civil war there.

Hagel told reporters in the United Arab Emirates that U.S. intelligence officials found that with “some degree of varying confidence,” Syrian President Bashar Assad’s regime had deployed chemical weapons on a “small scale.”

“It violates every convention of warfare,” Hagel added.

In a letter sent to Capitol Hill, the Obama administration said, “Our intelligence community does assess with varying degrees of confidence that the Syrian regime has used chemical weapons on a small scale in Syria, specifically the chemical agent sarin. This assessment is based in part on physiological samples. Our standard of evidence must build on these intelligence assessments as we seek to establish credible and corroborated facts. For example, the chain of custody is not clear, so we cannot confirm how the exposure occurred and under what conditions.”

The question now becomes: What next?

In the short-term, the Obama administration is calling for a United Nations investigation.

Obama has been hesitant to intervene in the more than two-year-civil war, condemning Assad’s actions but resisting calls for Americans to directly assist the rebels.

“Given the stakes involved, and what we have learned from our own recent experience, intelligent 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,” the letter to Sen. John McCain, R-Ariz., says.

The president was speaking at the dedication ceremony for President George W. Bush’s library in Dallas as the news broke. It is not yet clear if he intends to address the issue Thursday.