The change in Clinton's tone coincided with decidedly cool testimony from Defense Secretary Robert Gates on Wednesday before a House panel.
Gates said the U.S. military would enforce one if ordered to do so, but added that there's been "a lot of loose talk" about the military options regarding Libya.
"Lets' call a spade a spade," he said and listed off how creating an exclusion zone would require an attack on Libyan air defenses, which Mullen has said one had to assume were capable until proven otherwise. Gates also said Libya's airspace would require more planes to police than are aboard an aircraft carrier.
Earlier in the week, Gates and Joint Chiefs Chairman Adm. Michael Mullen argued that there are no confirmed reports that the Libyan Air Force has bombed within Libya.That was before the videos came out yesterday showing bombs being dropped near CNN reporters.
Gen. James Mattis, the commander of U.S. Central Command, which would oversee any U.S. involvement, was even more blunt in separate testimony before the Senate Armed Services Committee on Tuesday.
"My military opinion is, sir, it would be challenging. You would have to remove the air defense capability in order to establish the no-fly zone so it - no illusions here, it would be a military operation. It wouldn't simply be telling people not to fly airplanes," he said.
Pentagon and administration planners have pointed out the demands on U.S. resources of the war in Afghanistan and the winding down of its military presence in Iraq. Helping enforce an exclusion zone over Libya would require round-the-clock coverage from a host of military assets, according to Pentagon officials.
Plus, to give such military action international legitimacy, would require new authorization from the U.N. Security Council. There is some concern that Russia and China, each of which has the power to veto any Security Council resolution, would be skeptical of authorizing military action.
These roadblocks have not yet quieted calls on from both sides of the political aisle on Capitol Hill that a no-fly zone be pursued by the U.S. and NATO.
Speaking on MSNBC, Sen. Joe Lieberman, I-Conn., admitted that enforcing such a zone would be difficult to set up. But he said that should not preclude it.
"Is it complicated? Yeah. Can we do it? Of course, the U.S. and international NATO militaries can do it. We did it for a long time and quite successfully in Iraq. This would be for a much shorter time, I hope and pray," he said.
There is a perception among some on Capitol Hill that a no-fly zone would not be difficult to enforce.
"I really don't agree that the no-fly effort in Libya should be that difficult… I frankly think the threat of a no-fly zone, if we could put any group together so it wouldn't be solely an American effort, a United States effort, is worth vigorously pursuing, said Sen. Roy Blunt, R-Mo., on Capitol Hill in an exchange with Clinton Wednesday. "A tragedy is happening there now and you're speaking up on it and I appreciate that, but I would think we could do more."
With reporting by Jake Tapper, Sunlen Miller, Luis Martinez and Kirit Radia.