US Secretary of Defense Robert Gates and Chairman of the Joint Chiefs of Staff Adm. Mike Mullen testify on Libya on Capitol Hill in Washington March 31, 2011.[Photo/Agencies]

WASHINGTON - The US is about to pull its attack planes out of the international air campaign in Libya, hoping NATO and others can take up the slack.

The announcement Thursday drew incredulous reactions from some in Congress who wondered aloud why the Obama administration would bow out of a key element of the military strategy even as it was showing results.

"Odd," "troubling" and "unnerving" were among critical comments by senators pressing for an explanation of the announcement by Defense Secretary Robert Gates and Joint Chiefs chairman Adm. Mike Mullen that American combat missions will end Saturday.

"Your timing is exquisite," Republican Sen. John McCain said sarcastically, alluding to Gadhafi's military advances this week.

Gates and Mullen, in back-to-back appearances before the House of Representatives and Senate armed services committees, also forcefully argued against putting the US in the role of arming or training Libyan rebel forces, while suggesting it might be a job for Arab or other countries. The White House has said repeatedly that it has not ruled out arming the rebels, who have retreated pell-mell this week under the pressure of a renewed eastern offensive by Gadhafi's better-armed and better-trained ground troops.

"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 said.

The White House press secretary, Jay Carney, said he saw no contradiction between Gates' remarks and President Barack Obama's statement that "he has not ruled it in or out." As yet, none of Obama's top advisers have publicly advocated a significant expansion of the US role aiding the opposition.

Gates and Mullen were early skeptics of getting involved militarily in Libya, and Gates made clear Thursday that he still worries about the possibility of getting drawn into an open-ended and costly commitment. That explains in part his view that if the rebels are to receive foreign arms, that task _ and the training that would necessarily go with it _ should not be done by Americans.

Gates said no one should be surprised by the US combat air pullback, but he called the timing "unfortunate" in light of Gadhafi's battlefield gains. He noted that the air attacks are a central feature of the overall military strategy; over time they could degrade Gadhafi's firepower to a point that he would be unable to put down a renewed uprising by opposition forces, he said.

Mullen and Gates stressed that even though powerful combat aircraft like the side-firing AC-130 gunship and the A-10 Thunderbolt, used for close air support of friendly ground forces, will stop flying after Saturday, they will be on standby. Mullen said this means that if the rebels' situation become "dire enough," NATO's top commander could request help from the US aircraft. The US also has used Marine AV-8B Harrier attack jets as well as Air Force F-15 fighters and B-2 and B-1 long-range bombers.

As of Sunday, France, Britain and other NATO countries will handle the task of conducting airstrikes on Libyan military targets, Mullen said. The remaining US role will be support missions such as aerial refueling, search and rescue, and aerial reconnaissance. It was not immediately clear whether the US would continue attacks with Tomahawk cruise missiles, which have been fired regularly from Navy ships and submarines in the Mediterranean from the opening moments of the campaign on March 19.

Republican Sen. Lindsey Graham suggested the pullback might jeopardize congressional support for the Libya mission.

"The idea that the AC-130s and the A-10s and American air power is grounded unless the place goes to hell is just so unnerving that I can't express it adequately," Graham said. "The only thing I would ask is, please reconsider that."

Asked by Democratic Sen. Jeanne Shaheen whether he was confident that NATO could sustain airstrikes alone, Gates replied, "They certainly have made that commitment, and we will see."

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. Gates defended it, asserting that a potential humanitarian disaster was averted when the US-led intervention stopped Gadhafi's forces as they closed in on Beghazi, the de facto rebel capital in eastern Libya. Gadhafi's forces initially were driven back, but they have since regained their lost ground.

Mullen revealed that a major factor in Gadhafi's ability to drive back the rebels _ essentially eliminating the territorial gains they had made last week with the help of international air strikes _ was bad weather. He said it grounded most combat missions earlier this week.

Obama had made clear that once US air power silenced Gadhafi's air defenses, permitting the establishment of a no-fly zone over the North African country, the US would reduce its role and let NATO take the lead. On Thursday, NATO assumed control of all aspects of the international campaign _ including enforcing the no-fly zone and attacking Gadhafi's military.

The US now finds itself in the unusual position of a back-seat partner in the Libya operation, with no clear path to empowering the rebels. A retired Army general, James Dubick, wrote Thursday in a war commentary that a necessary next step is to place NATO combat air controllers on the ground _ to include Americans _ to precisely direct air power. Trainers also are needed, he wrote.

"Right now, they (the rebels) are more like 'guys with guns' than an organized force and they need help," Dubick wrote. He is a former commander of US training mission in Iraq and is now a senior fellow at the Institute for the Study of War, a think tank.

Mullen said Gadhafi's army had lost as much as 25 percent of its firepower, although his ground forces still outnumber the rebels by about 10-to-1.