WASHINGTON (AP) -- Despite rebel setbacks and an increasingly public rift with NATO allies, the U.S. will stick to its plan to remain in the back seat of the Libya air campaign, the Obama administration insisted Tuesday after three weeks of air missions that have failed to turn the tide against Moammar Gadhafi.

France's defense minister declared that without full American participation, the West probably would not be able to stop attacks by Gadhafi loyalists on besieged rebel cities.

U.S. officials said they were comfortable with their role and had no plans to step up involvement, even as British and French officials said Washington's military might was needed to ensure the mission's success. The Americans said NATO could carry out the operation without a resumption of the heavy U.S. efforts that kicked it off last month.

"The president and this administration believes that NATO, and the coalition of which we remain a partner, is capable of fulfilling that mission of enforcing the no-fly zone, enforcing the arms embargo and providing civilian protection," White House spokesman Jay Carney told reporters.

"The U.S. has not abandoned this operation by any means," State Department spokesman Mark Toner said. "We still are offering support where we can. I don't think it's correct to say that there's somehow discord in the alliance."

The public complaints of Britain and France, however, contradicted that position, and U.S. officials contended privately that some in Europe appeared to be backing down on pledges to take the lead in the operation once the opening phase was over. The administration had not wanted to keep a primary role after that point and had made its participation in the NATO mission contingent on having only a supporting function afterward.

With the disagreement out in the open, Secretary of State Hillary Rodham Clinton is expected to hear loud calls for the U.S. to resume heavier fighting when she travels to Germany for meetings of NATO foreign ministers on Thursday and Friday. Those talks are expected to be dominated by the situation in Libya, where rebels fighting forces loyal to Gadhafi are facing increasing challenges and appealing for additional assistance.

At the State Department, spokesman Toner said President Barack Obama had been clear from the beginning that the U.S. "role would diminish as NATO stepped up and took command and control of the operation."

He added, "The U.S., of course, as needed, would help out if requested in other capacities, in other capabilities, but really our role has receded in this mission."

At the Pentagon, Marine Col. Dave Lapan said there was no move to increase American military involvement.

"I don't see any planning to re-assert U.S. strike aircraft and forces as we saw early in the campaign," the Pentagon spokesman said. "NATO has those capabilities to conduct strikes."

"Ultimately, what needs to happen is Gadhafi needs to stop attacking his own people," Lapan said. "The lack of U.S. strike missions doesn't change that."

At NATO headquarters in Brussels, alliance officials agreed and said the operation was succeeding.

NATO Brig. Gen. Mark Van Uhm rejected criticism of the operation. He said the North Atlantic military alliance was performing well in enforcing the arms embargo, patrolling the no-fly zone over Libya and protecting civilians.

"With the assets we have, we're doing a great job," Van Uhm told reporters.

France and Britain differed, calling for the rest of the group, in particular the United States, to step up the campaign.

At a European Union meeting in Luxembourg, Paris lamented the limited U.S. military role in Libya and chided Germany, too, for its lack of involvement. In a dire analysis, France's defense minister said that without full American participation in the combat operation, the West probably couldn't stop Gadhafi's attacks on rebel-held cities.

French Foreign Minister Alain Juppe shredded NATO's united front, saying its actions to this point were "not enough" to ease the pressure on the city of Misrata, which has been subjected to weeks of bombardment by forces loyal to Gadhafi.

"NATO absolutely wanted to lead this operation. Well, voila, this is where we are," Juppe said. "It is unacceptable that Misrata can continue to be bombed by Gadhafi's troops."

Britain's Foreign Secretary William Hague agreed that the allies must "intensify" their efforts, but he used a more diplomatic tone.

"The U.K. has in the last week supplied additional aircraft capable of striking ground targets threatening the civilian population of Libya," Hague said before a meeting of EU foreign ministers. "Of course, it will be welcome if other countries also do the same. There is always more to do."

French Defense Minister Gerard Longuet complained that France and Britain were carrying "the brunt of the burden." He said the reduced U.S. effort - American forces are now in support, not combat, roles in the airstrike campaign - have made it impossible "to loosen the noose around Misrata," which has become a symbol of the resistance against Gadhafi.

Longuet also criticized Germany, which is not taking part in the military operation, saying that Berlin's commitment to primarily back a humanitarian effort only was "secondary" at best. Germany does not take part in NATO's military airstrikes in Libya because it sees the operation as too risky. Italy also has been reluctant to get involved in the airstrikes because it had been Libya's colonial ruler.

France's frustration with the stalemate on the ground, where Libyan rebels have struggled to capitalize on Western air attacks, has been echoed in several Western capitals, but rarely were the comments as barbed as Juppe's.

The reduced U.S. role since NATO took over command on March 31 has clearly affected the operation.

"Let's be realistic. The fact that the U.S. has left the sort of the kinetic part of the air operation has had a sizable impact. That is fairly obvious," said Swedish Foreign Minister Carl Bildt.

Libyan opposition spokesman Ali al-Issawi said that Gadhafi's soldiers have killed about 10,000 people throughout the country and injured 30,000 others, with 7,000 of the injured facing life-threatening wounds. He said an additional 20,000 people were missing and suspected of being in Gadhafi's prisons. There was no way to independently verify his claims.

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Casert reported from Luxembourg. Associated Press writers Pauline Jelinek and Sagar Meghani contributed from Washington, Angela Charlton from Paris, Selcan Hacaoglu from Turkey, Adam Schreck from Doha, Qatar, and Paisley Dodds and Raphael G. Satter from London.