An operation billed as a humanitarian intervention in Libya by President Obama was described in starkly more military terms Thursday by the administration’s top two defense officials.

Defense Secretary Robert M. Gates told lawmakers that continuing coalition attacks on Libyan government troops — even when they were not directly threatening civilians — would encourage senior government and military officials to break with Libyan leader Moammar Gaddafi and inspire more civilians to join opposition forces.

“His military, at a certain point, is going to have to face the question of whether they are prepared over time to be destroyed by these air attacks or whether they decide it’s time for him to go,” Gates told the Senate Armed Services Committee.

The testimony came amid reports that another member of Gaddafi’s inner circle had defected, boosting the spirits of the beleaguered rebels. A top Libyan Foreign Ministry official, Ali Abdel Salam al-Treki, announced his defection in a statement sent to news agencies by his nephew. British Prime Minister David Cameron and White House spokesman Jay Carney on Thursday hailed the earlier defection of Libyan Foreign Minister Musa Kusa, saying it was a sign that Gaddafi’s power was eroding.

In eastern Libya, rebels fought their way back into the key oil-refinery town of Brega but were soon forced to withdraw under heavy shelling from Gaddafi’s forces, which maintain a huge firepower advantage over the ragtag opposition army.

In his testimony, Adm. Mike Mullen, the chairman of the Joint Chiefs of Staff, blamed the rebels’ losses in recent days on the heavy cloud cover in Libya, which has prevented U.S. and allied jets from attacking Gaddafi’s ground forces.

Mullen said the sustained bombing campaign had destroyed as much as 25 percent of Gaddafi’s military arsenal and pledged that coalition forces would continue to hammer away at his ground forces.

Some of the United States’ partners have acknowledged that the initial descriptions of the intervention in Libya no longer apply. “What is happening in Libya is not a no-fly zone,” a senior European diplomat told reporters, speaking on the customary condition of anonymity. “The no-fly zone was a diplomatic thing, to get the Arabs on board. What we have in Libya is more than that.”

Although Gates said that unseating Gaddafi was not the stated goal of the military mission, he made clear that the United States and its allies intended to use military force to aid the Libyan opposition and compel Gaddafi and his inner circle to surrender.

“I mean we’re blowing up [Gaddafi’s] ammunition supplies,” Gates said in separate testimony before the House. “He can’t resupply from abroad . . . over time, that should work to the advantage of those in the opposition.”

The defense secretary also made clear his preference that other nations take the lead in training and arming the Libyan rebels. “If there is going to be that kind of assistance to the opposition, there are plenty of sources for it other than the United States,” he said. French officials have been among the most aggressive in pressing for military aid to the rebels.

In the House, lawmakers from both parties berated Gates over the cost of the mission, its undetermined length and what they said were its fuzzy goals. In the Senate, there was more support for the intervention, with several lawmakers saying they worried that the United States was not doing enough to oust Gaddafi.

The militaristic language used by Gates and Mullen prompted some lawmakers who oppose the intervention to accuse the Obama administration of misleading the public about its aims in Libya. “It seems to me, and I think everybody else, that we are clearly involved in regime change,” said Sen. James Webb (D-Va.).

Rep. Mike Coffman (R-Colo.) said the administration’s efforts to label the Libya mission a humanitarian intervention were dishonest. “This is the most muddled definition of an operation probably in U.S. military history,” he said. “To say this is not about regime change is crazy. Of course, it is about regime change.”

Gates acknowledged that the U.N. Security Council resolution approving a no-fly zone over Libya had authorized only the interdiction of Libyan aircraft, the enforcement of an arms embargo and the use of military force to protect civilians. But he defended the efforts to degrade Gaddafi’s military as a means of protecting the Libyan people from further attacks by the government.

The CIA has told lawmakers in classified briefings that Gaddafi’s forces have moved into towns where there are no reporters and killed a “substantial number” of people, said Sen. Dianne Feinstein (D-Calif.), chairwoman of the Senate Select Intelligence Committee, on Thursday. She advocated expanding the U.S. objectives to include Gaddafi’s ouster.

Mullen and Gates said that the U.S. level of participation in the coalition would begin to decline significantly in the coming days with NATO taking over command. In the Senate, several lawmakers voiced concern that reducing the U.S. role would lessen the effectiveness of the attacks on Gaddafi’s ground forces. They pressed Mullen and Gates for assurances that American AC-130 aircraft, which are among the most effective weapons in the U.S. arsenal against ground forces, would remain in the fight.

“I just can’t understand how we’re going to meet the objective that you’ve identified without going forward in a forceful fashion than we are right now,” said Sen. Kelly Ayotte (R-N.H.).

In the House, mainly Republican lawmakers complained that the United States did not know enough about the rebel forces challenging Gaddafi’s decades-long rule. “We may not know much about the opposition or the rebels, but we know a great deal about Gaddafi,” Gates replied tersely. “This guy has been a huge problem for the United States for a long time.”

A senior administration official said the congressional criticism reflected the lack of options.

“There are appropriately a lot of questions being raised, but apart from those who want to go further and impose regime change militarily, there are not a lot of alternatives put forward,” said the official, speaking on the condition of anonymity. “It’s a situation that involves bad choices and worse choices.”

Staff writers Mary Beth Sheridan and Greg Miller contributed to this report.