The announcement, in the hope that Nato partners can take up the slack, drew incredulous reactions.
The announcement drew incredulous reactions from some in Congress who wondered aloud why the Obama administration would bow out of a key element of the strategy for protecting Libyan civilians and crippling Muammar Gaddafi's army.
"Odd," "troubling" and "unnerving" were among critical comments by senators pressing for an explanation of the announcement by Defence Secretary Robert Gates and Joint Chiefs chairman Admiral Mike Mullen that American combat missions will end on Saturday.
"Your timing is exquisite," Republican Sen. John McCain said sarcastically, alluding to Gaddafi's military advances this week.
Gates and Mullen, in 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, forced to retreat this week under a renewed eastern offensive by Gaddafi'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.
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.
As of Sunday, France, Britain and other Nato countries will handle the task of conducting air strikes on Libyan military targets, Mullen said. The remaining US role will be support missions such as aerial refuelling, 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.
Republican Sen. Lindsey Graham suggested the pullback might jeopardise 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 air strikes alone, Gates replied, "They certainly have made that commitment, and we will see."
Mullen revealed that a major factor in Gaddafi's ability to drive back the rebels was bad weather. He said it grounded most combat missions earlier this week.Reuse content