U.S. and allied intelligence have detected Syrian movement of chemical weapons components in recent days, a senior U.S. defense official said Monday, as the Obama administration strongly warned the Assad regime against using them.

A senior defense official said intelligence officials have detected activity around more than one of Syria's chemical weapons sites in the last week. The defense official spoke on condition of anonymity because he was not authorized to speak publicly about intelligence matters.

Secretary of State Hillary Rodham Clinton, in Prague for meetings with Czech officials, reiterated President Barack Obama's declaration that Syrian action on chemical weapons was a "red line" for the United States that would prompt action.