Related: 58 percent say Britain is wrong to intervene

One of the more vocal congressional skeptics of the Libya action, Sen. Richard Lugar, called Monday for full debate on the objectives and costs of the conflict and a need for a declaration of war.

"There needs to be a plan about what happens after Gadhafi, who is in charge then, and who pays for this all," said Lugar, a Republican from Indiana. "With the Arab League having second thoughts and Turkey nixing NATO taking over, there are even more questions. We also have to debate how all this affects the Saudis, Bahrain and Yemen."

Gates, speaking in St. Petersburg, Russia, said the US military's role would soon shrink.

"I expect us very soon to recede back into a supporting role with other nations carrying a significant proportion of the burden in implementing and enforcing the no-fly zone," the Pentagon chief said. "And the president has made very clear the United States will not put any forces in Libya, on Libyan soil."

Ham foresaw potential new complications for the US and its coalition partners: how to respond in the event that rebel forces seeking Gadhafi's overthrow launch their own offensive in areas where civilians are threatened. Would the UN mandate for protection of civilians require coalition attacks on the rebels in that case?

Ham noted that while some of the rebels are ordinary civilians trying to protect their homes, families and businesses from Gadhafi's forces, others have taken up heavy arms and are mobilized in armored vehicles.

"So this will become a particular challenge for us should that eventuality (a rebel offensive) occur," Ham said. He repeatedly stressed that the US military has not and will not intervene on the side of the rebels; he said there has been no military communication or coordination with rebel leaders.