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Carolyn Kaster, Associated Press
Defense Secretary Robert Gates,, left, and Joint Chiefs Chairman Adm. Michael Mullen arrive on Capitol Hill in Washington, Thursday, March 31, 2011, to testify before the House Armed Services Committee hearing on military operations in Libya.

WASHINGTON — As the U.S. debates its future participation in the Libyan conflict, defense officials slammed the brakes Thursday on any major American role aiding opposition groups and insisted that the Obama administration should not be the one to arm the rebels.

Defense Secretary Robert Gates and Adm. Mike Mullen, the chairman of the Joint Chiefs of Staff, said that if the rebels are to get arms, money and training, countries other than the U.S. should provide that assistance. The military leaders told Congress the rebels remain a largely unknown quantity, but are a better alternative than Col. Moammar Gadhafi.

The White House said again Thursday that arming the rebels is under consideration.

"My view would be, 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," Gates told the House Armed Services Committee.

Amid reports that the CIA has small teams working with the rebels in Libya, Gates insisted there will be no U.S. military ground forces in Libya "as long as I am in this job."

He was circumspect, if less definitive, about U.S. involvement in lesser forms of help for the rebels such as military training or weaponry.

The U.S. turned over control of the military operation to NATO Thursday, just hours before Gates and Mullen told Congress that future U.S. participation will be limited and will not involve a major role in airstrikes as time goes on.

They were unable, however, to answer key questions from clearly agitated lawmakers about the length of the operation and how it will play out if Gadhafi does not relinquish power.

Many lawmakers were angered by what they said was the administration's lack of candor with Congress ahead of the Libya mission. Several complained that the mission is expensive and ill-defined.

While Gates defended the U.S. intervention, reminding lawmakers that Gadhafi has been a persistent and dangerous enemy, he also acknowledged that efforts to oust Gadhafi may not work.

"You could have a situation where you achieve the military goal and not achieve the political goal" of regime change, Gates said. He added that the U.S. had considered the possibility of a prolonged stalemate, and although he said the United States could not accept a reorganized Libyan government with Gadhafi in power, he steered around the question of what the U.S. could do to prevent that.

The defense leaders struggled to avoid being dragged into the increasingly bitter conflict between Congress and the White House over authorization for the military operation.

In fact, Obama gathered congressional leaders to the White House and by telephone the day before the mission began to inform them of his decision. The Senate also unanimously approved a resolution March 1 backing the no-fly zone.

Responding to the complaints, Deputy Secretary of State Jim Steinberg told a separate House panel on Thursday that "stopping a potential humanitarian disaster of massive proportions became a question of hours, not days. And so we acted decisively to prevent a potential massacre."

Gates downplayed the threat of a terrorism upsurge flowing from the wave of Mideast unrest.

Gates and Mullen said Gadhafi's military has been degraded by as much as 25 percent, but Mullen noted that Gadhafi's forces still outnumber the rebels by about 10-to-1.

Gates said that he believes political and economic pressures eventually will drive Gadhafi from power, but the military operation will help force him to make those choices by degrading his defense capabilities.

In the long run, he said, al-Qaida "is a loser in this revolution that is taking place."

Separately, the State Department said the U.S. was not involved in the defection of Gadhafi's top diplomat, Moussa Koussa, although a U.S. diplomat had talked with Koussa.

"He's obviously been a part of the Gadhafi regime for many, many years," State Department Mark Toner said. "I obviously don't want to talk about what conversations we may be having with him and what kind of intelligence we may be able to gather from him, but he certainly has a wealth of information to share, should he decide to."

Associated Press writers Adam Goldman and Robert Burns contributed to this report.