U.S. intelligence agencies now believe that Syrian President Bashar Assad's regime has used chemical weapons in its struggle to hold onto power, Defense Secretary Chuck Hagel said today.

Hagel, speaking to reporters in the United Arab Emirates, said the White House sent a letter to members of Congress this morning disclosing that intelligence agencies had made that assessment, which followed a series of similar conclusions reached by Britain, France and most recently Israel.

President Obama has termed the use of chemical weapons a "red line" that could trigger military intervention to secure Syria's large stockpile of chemical weapons, a move that few U.S. officials are eager to make.