The White House was on the defensive Monday over its handling of the Syrian civil war, pushing back at critics who questioned President Obama's "red line" ultimatum to the Syrian government, the possible use of chemical weapons by anti-government rebels and whether Israel's airstrikes inside Syria require greater response from the U.S.

Even people inside the administration are raising doubts about the president's handling of the growing crisis, telling the New York Times that the president misspoke when he threatened U.S. intervention if the Syrian government crossed a "red line," something it was said to do by using chemical weapons against its own people.

But the White House insists that the president had chosen his words carefully in addressing Syria even if he hadn't spelled out what the response would be.

"What he never did -- and it is simplistic to do so -- is to say that if X happens, Y will happen," White House press secretary Jay Carney said. "He has never said what reaction he would take at a policy level to the proved crossing of the red line in Syria, simply that he would consider it a red line that had been crossed."

Even the president's liberal supporters are frustrated.

"I can't really explain it," said one Democratic consultant with close ties to the White House. "One day, administration officials say one thing, the next day, the exact opposite. It's not exactly their finest moment."

With Israel initiating attacks, Obama faces a tough call on whether to order similar U.S. air attacks or to create a no-fly zone over Syria, much as he had in Libya.

"Syria does not have strong air defenses by U.S. standards, but it is still very large," said Anthony Cordesman, an expert on Syria at the Center for Strategic and International Studies. "It would take a major U.S. air effort to accomplish quickly and the United States might well take some losses if Syria fought back and would have to have a sustained presence if Syria chose not to fight."

The heightened calls for action are also coming from some within Obama's own party.

Sen. Robert Menendez, D-N.J., introduced a bill on Monday calling for the U.S. to arm the anti-government Syrian rebels, something Obama has so far resisted.

"The Assad regime has crossed a red line that forces us to consider all options," Menendez said. "The greatest humanitarian crisis in the world is unfolding in and around Syria, and the U.S. must play a role in tipping the scales toward opposition groups and working to build a free Syria."

Complicating matters for Obama was a United Nations investigator who suggested the rebels themselves had used chemical weapons during the two-year civil war. Obama had called for a U.N. investigation in Syria before deciding how to respond.

The White House was not moved by the probe's early findings, which could undercut a central premise of Obama's message -- that rebels lacked the capability to use such weapons of mass destruction.

"We find it highly likely that chemical weapons, if they were in fact used in Syria -- and there is certainly evidence that they were -- that the Assad regime was responsible," Carney insisted.