Top-ranking lawmakers on both sides of the aisle declared Thursday that the "red line" in Syria has been crossed, calling for "strong" U.S. and international intervention after administration officials revealed the intelligence community believes chemical weapons were used.

Sen. John McCain, R-Ariz., and Sen. Dianne Feinstein, D-Calif., chairwoman of the Senate intelligence committee, were among those urging swift action.

McCain, who has long called for more involvement in Syria, voiced concern that the administration would use "caveats" to avoid acting on the new intelligence. He said America's enemies are paying "close attention" to whether the U.S. follows through, as the White House signaled it wanted to see more proof before responding to the new information.

"I worry that the president and the administration will use these caveats as an excuse not to act right away or act at all," McCain told Fox News. "The president clearly stated that it was a red line and that it couldn't be crossed without the United States taking vigorous action."

He called for the U.S. to help establish a no-fly zone and "safe zone" in Syria, as well as provide weapons to the "right people."

Defense Secretary Chuck Hagel first revealed 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 then 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."

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

The White House, however, 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."

A White House official also urged caution, invoking the Iraq war as an example of why the administration should be absolutely certain before going forward....