Washington ? The U.S. should avoid developing a closer relationship with Libyan opposition forces, defense leaders said Thursday, telling an often hostile Congress that foreign nations must now take over airstrike responsibilities and any effort to train and equip the rebels.

With the U.S. role in Libya at a turning point, the next critical decision is how, if at all, the U.S. chooses to support the opposition forces, particularly in the face of the ongoing budget crisis at home. Defense Secretary Robert Gates said he is opposed to arming the rebels, a step his boss President Barack Obama has not ruled out.

Gates and Adm. Mike Mullen, the chairman of the Joint Chiefs of Staff said it was time to turn the bulk of the conflict over to NATO.

The U.S. turned over control of the military operation to NATO Thursday, just hours before Gates and Mullen addressed Congress.

“The question of what kind of assistance to provide to the opposition is clearly the next step in terms of nonlethal or weapons,” Gates told senators. “All the members of the coalition are thinking about that at this point, but as with our government, no decisions have been made.”

Gates and Mullen said that if the rebels are to get arms and training, countries other than the U.S. should provide that assistance. And they stressed that the U.S. still does not have enough good information about who the disparate opposition forces are.

“My view would be, if there is going to be that kind of (training) assistance to the opposition, there are plenty of sources for it other than the United States,” Gates told the House Armed Services Committee.

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. The ongoing scope of U.S. military action drew heated debate among senators unhappy that the Pentagon will no longer be conducting airstrikes in the coming days — leaving that key combat responsibility to allies such as the French, British and Canadians.