Defense Secretary Chuck Hagel said Thursday the U.S. intelligence community believes the Syrian regime used the chemical weapon sarin, a revelation that immediately raised the question of whether a "red line" had been crossed in the country's civil war.

Hagel confirmed the intelligence assessment, which was detailed in a letter to select members of Congress, while speaking to reporters on a visit to Abu Dhabi. The administration swiftly released those letters, which said U.S. intelligence determined with varying degrees of confidence that "the Syrian regime has used chemical weapons on a small scale in Syria, specifically the chemical agent sarin."

The White House stressed that this was not enough to confirm how the nerve gas was released -- though acknowledged it is "very likely" to have originated with the regime of Bashar Assad -- and pressed the United Nations for a "comprehensive" investigation. The letter from the White House director of the Office of Legislative Affairs to leading members of the Senate Armed Services Committee said the assessment was based in part on "physiological samples."

Secretary of State John Kerry further confirmed that there were two documented instances of chemical weapons use.

The assessment is likely to prompt calls for more serious consideration of intervention. President Obama has said the use of chemical weapons would be a "game-changer" in the U.S. position on intervening in the two-year-old Syrian civil war. Obama said last August that "a red line for us" would be the movement or use of chemical weapons, adding "that would change my calculus."

Sen. John McCain, R-Ariz., suggested Thursday the "red line" had been crossed. Sen. Bob Corker, R-Tenn., also said the assessment is "deeply troubling and, if correct, means that President Obama's red line has certainly been crossed."