The move, not yet publicly announced, reinforces the White House message of a diminishing U.S. role - a central point in President Barack Obama's national address Monday night on Libya. The White House booked Defense Secretary Robert Gates and Secretary of State Hillary Rodham Clinton on three Sunday news shows to promote the administration's case ahead of the speech.
Yet Gates, asked whether the military operation might be over by year's end, said, "I don't think anybody knows the answer to that."
At least one of the five Navy ships and submarines that have launched dozens of Tomahawk cruise missiles at Libyan targets from positions in the Mediterranean Sea has left the area, three defense officials said. They spoke on condition of anonymity in order to discuss sensitive military movements.
That still leaves what officials believe is sufficient naval firepower off Libya's coast, and it coincides with NATO's decision Sunday to take over command and control of the entire Libya operation. Aided by international air power, Libyan rebels were reported to have made important gains by capturing two oil complexes along the coast.
The shrinking of the naval presence adds substance to Obama's expected reassurance to the American people that after kicking off the Libyan mission, the U.S. is now handing off to partner countries in Europe and elsewhere the bulk of the responsibility for suppressing Gadhafi's forces.
NATO's governing body, meeting in Brussels, accepted a plan for the transfer of command. That is expected to mean that U.S. Army Gen. Carter Ham, who has been the top commander of the Libya operation, will switch to a support role.
Obama administration officials claimed progress in Libya, but lawmakers in both parties voiced skepticism over the length, scope and costs of the mission.
Obama is trying to address those issues in a speech that's expected to provide his fullest explanation of the U.S. role in Libya and what lies ahead.
Sen. Richard Lugar, R-Ind., questioned whether it made sense to be involved at all. "I don't believe we should be engaged in Libyan civil war," Lugar said on NBC's "Meet the Press."
"I believe the Libyans are going to have to work that out. The fact is that we don't have particular ties with anybody in the Libyan picture."
Sen. Carl Levin, D-Mich., the Senate Armed Services Committee chairman, was broadly supportive of the president's steps so far. "It is a flyover which is succeeding. It has set Gadhafi back. He's on his heels now," Levin said on CNN's "State of the Union."
Still, Levin said it was unclear how long the air campaign will have to last if Gadhafi clings to power.
Gates, an early skeptic of establishing a no-fly zone, told ABC's "This Week" that for practical purposes, the establishment of the zone is complete and can now be sustained "with a lot less effort than what it took to set it up."
In advance of Obama's speech at 7:30 p.m. EDT Monday, Gates and Clinton stressed the administration's message that the U.S. role in the mission will shrink, illustrating that it's possible for the U.S. military to partner with others without always being the leader.
Gates said the no-fly zone and efforts to protect civilians from attack by pro-Gadhafi forces will have to be sustained "for some period of time."
Among other hard questions for Obama is whether the Libyan intervention should serve as a model for U.S. policy toward other Arab countries where revolts against authoritarian governments are gaining ground, including Syria and Yemen, and where civilians are at risk of violent reprisals.
Clinton declined to say if the U.S. might be willing to enter other such conflicts. She said it was too early to talk of getting involved in Syria, where security forces have opened fire on protesters amid nationwide unrest. Unlike Gadhafi, Syrian President Bashar Assad is a "different leader" and many members of Congress who have visited the country "believe he's a reformer," Clinton said.
Clinton and Gates insisted that the objective in Libya was limited to protecting civilians, even as they hoped the pressure of concerted international penalties and isolation might strip away Gadhafi's remaining loyalists and cause his government to crumble.
"One should not underestimate the possibility of the regime itself cracking," Gates said.
Asked if the Libyan conflict posed a threat to the United States, Gates said it was "not a vital national interest" but he insisted that the situation nevertheless demanded U.S. involvement. With tenuous democratic transitions under way in the neighboring countries of Tunisia and - more important to the U.S. - Egypt, allowing the entire region to be destabilized was a dangerous option.
Citing military gains against Libya over the past week, Gates said Pentagon officials are now planning the start of a force reduction. He was not specific, but he appeared to refer to moving some of the dozens of American ships or aircraft - or both - out of the immediate area.
"We will begin diminishing the level of our engagement, the level of resources we have involved in this," he said, adding that as long as there is a no-fly zone, "we will continue to have a presence." He gave as examples U.S. surveillance and reconnaissance aircraft that support the no-fly zone.
Even as naval firepower was reduced, Pentagon officials said they were considering adding air power. Vice Adm. William Gortney told reporters on Friday that low-flying Air Force AC-130 gunships, armed drones and helicopters were among weaponry that might be deployed to provide more precise air power against Libyan ground forces battling in urban areas. High-flying fighter jets run a high risk of causing civilian casualties if they attack inside cities.
It is unclear how long the U.S. will keep a Navy command ship, the USS Mount Whitney, in its role as overall coordinator of the sea and air campaign, once NATO assumes full command. NATO could run the full operation out of its Allied Joint Forces Command headquarters in Naples, Italy.
The Navy has had three submarines in the Mediterranean - the USS Providence, the USS Scranton and the USS Florida - plus two destroyers, the USS Barry and the USS Stout. All five are equipped with Tomahawks, the cruise missile that can fly long distances and maneuver to hit fixed targets like surface-to-air missile batteries and other air defense elements that posed a threat to coalition air patrols. It was not clear Sunday which of the five had been ordered out of the area.
Through the first seven says of the campaign to ground Gadhafi's air force, those American ships and subs launched 184 Tomahawks - more than half of them in the opening moments of the assault on March 19, according to figures provided by the Pentagon. None was launched Saturday and two on Sunday, bring the U.S. total to 186. That is in addition to seven cruise missiles fired by British warships.
Gates and Clinton taped interviews Saturday on NBC, ABC and CBS' "Face the Nation" that aired Sunday.
Associated Press writer Tom Raum contributed to this report.