ABU DHABI, United Arab Emirates – U.S. intelligence has determined that embattled Syrian President Bashar Assad has deployed chemical weapons to subdue the rebellion in his country, Obama administration officials said Thursday, a disclosure that could move the United States closer to intervening in that country’s 2-year-old civil war.

The White House informed members of Congress in a letter that intelligence agencies believe Assad’s regime has used sarin, a nerve agent, “on a small scale in Syria.”

It marked the first time the United States has accused Assad of using chemical weapons, which President Obama has termed a “red line” that would trigger unspecified U.S. action.

The White House is deeply reluctant to become embroiled in the Syrian conflict, but after Britain, France and Israel in recent weeks accused Assad of using chemical weapons, it raised pressure on the United States to finalize its own assessment.

For days, Obama administration officials said that intelligence agencies hadn’t reached a conclusion, but Defense Secretary Chuck Hagel, traveling Thursday in the United Arab Emirates, said the positive determination was made “within the past 24 hours.”

The White House said the assessment was based partly on physiological samples, a possible reference to soil and tissue evidence that Israel and others cited. It didn’t specify when and where the alleged sarin use took place, but Israel’s top military intelligence analyst said this week that a sarin-based nerve agent was used in attacks March 19 near Aleppo and Damascus.

The U.S. intelligence community has “varying degrees of confidence” in the findings, the letter said, signaling that there are disagreements within the government not only about the quality of the evidence but also about how to respond. Hagel said intelligence officials were continuing to gather evidence and that the administration was pushing for a United Nations investigation.

Syria’s chemical weapons stockpile is believed to be one of the world’s largest, and Obama administration officials have said repeatedly that use of the chemical agents — or transferring them to terrorist groups — would cross a “red line.” For months, the Pentagon has been drawing up options in case the administration decided to intervene to secure the stockpiles, and last week Hagel announced that 200 Army troops would be deployed to neighboring Jordan to assist in the planning.