Thirteen days after the United States turned over command of the Libya campaign, the NATO alliance showed signs of strain Tuesday, with France and Britain complaining that their partners are not doing enough to help protect rebel-held cities from assaults by Moammar Gaddafi’s troops.

The unusual open criticism, from French Foreign Minister Alain Juppe and British Foreign Secretary William Hague, raised questions about the ability of the 28-member alliance to smoothly conduct a large-scale, sustained military operation without the United States playing a commanding role, as it has in the past.

“The Americans have the numbers of planes, and the Americans have the right equipment,” said Francois Heisbourg, a military specialist at the Foundation for Strategic Research in Paris.

But the Obama administration appeared to reject the possibility of an expanded U.S. military role in the near future. A State Department spokesman insisted Tuesday that NATO was performing adequately in enforcing the no-fly zone, and said the alliance was fine-tuning its tactics to address complaints about the campaign’s effectiveness.

“We have every confidence in NATO’s ability to carry out the task of enforcing the arms embargo as well as the no-fly zone and the protection of civilians in Libya,” the spokesman, Mark Toner, told reporters in Washington.

“The U.S., of course, as needed, would help out if requested in other capacities. But, really, our role has receded,” he said.

Juppe and Hague, in separate comments, urged more NATO countries to dispatch aircraft ready to participate actively in the bombing operations. A European official who spoke on the condition of anonymity because of the sensitivity of the issue said Hague was speaking in particular of restrictions imposed by most contributing governments that have limited many NATO warplanes to secondary roles.

“We must maintain and expand our efforts in NATO,” Hague said upon arriving in Luxembourg for a European Union meeting. “That is why the United Kingdom in the last weeks supplied additional aircraft capable of striking ground targets that threaten the civilian population. Of course, it would be welcome if other countries did the same.”

Juppe, in even more direct remarks on France Info radio before departing for the Luxembourg meeting, also said NATO must increase the effectiveness of its bombing operations. That would help prevent Gaddafi’s forces from shelling the besieged city of Misurata and other contested cities.

“NATO wanted to take over military operations, and we accepted that,” Juppe said. “But it must play its full role. That is to say, it must prevent Gaddafi from using heavy weapons against the civilian population.”

What NATO is doing now, he added, “is not sufficient.”

‘Not enough aircraft’

Seventeen nations, with about 175 planes, have been officially participating in the NATO campaign since the United States stepped back March 31. Of those, only France and Britain are allowing their aircraft to fly without restrictions on their use, noted Jean-Pierre Maulny, a defense expert at the International and Strategic Relations Institute in Paris. Of the other aircraft, some have been barred from bombing, others from hitting vehicles and others from flying attack missions at all, according to reports from Brussels.

But Brig. Gen. Mark van Uhm, NATO’s Dutch chief of allied operations, told reporters at NATO headquarters in Brussels that bombing operations have not dropped off since the United States handed over the command and withdrew most of its aircraft from the campaign. Over the past week, he said, the number of sorties — one flight by one plane — has averaged 155, of which 62 were strike missions.

Seeking to refute criticism that the campaign has become ineffective in protecting rebel-held cities, van Uhm said NATO aircraft destroyed 49 Libyan army tanks and nine armored personnel carriers over the weekend alone, most of them near the contested eastern city of Ajdabiya. The airstrikes were credited by rebels with helping halt a Libyan army assault on the strategic crossroads town, 99 miles south of rebel headquarters in Benghazi.

“I think with the assets we have, we are doing a great job,” he said.

NATO’s performance in running the air campaign was expected to be a key topic at a meeting of NATO foreign ministers in Germany later this week. Secretary of State Hillary Rodham Clinton was scheduled to fly to Berlin on Wednesday for two days of talks, while the department’s top Middle East experts huddled with European and Arab counterparts and Libyan opposition leaders in the Persian Gulf state of Qatar to map out strategy.

Some NATO officials have suggested that their bombing would be more effective if the U.S. A-10 Warthog close-support plane were made available again, Heisbourg said, because none of NATO’s other air forces possess similar slow-flying warplanes.

“The essence of the problem,” he added, “is not enough aircraft and not enough of the right kind of aircraft.”

The pilots of France’s Rafale and Mirage 2000 fighter-bombers, for instance, have found their Sagem AASM 125 air-to-ground missile systems to be remarkably effective in striking Libyan tanks, he noted. But the problem is that they are fired from high up and Gaddafi’s forces have adopted the same ragtag uniforms and four-wheel-drive pickups as the rebels to blend into the desert landscape.

“If it’s a tank down there, it’s okay,” Heisbourg said. But the missiles “certainly can’t tell the difference between a rebel Toyota pickup and a loyalist Toyota pickup,” he added.

French objections

Juppe’s comments carried a particular edge because French President Nicolas Sarkozy’s government strongly opposed turning operations over to a NATO command after the Obama administration decided to reduce its commitment at the end of March.

The ostensible French argument was that Arab governments would oppose any operation carried out with the NATO brand name. But its unsaid goal was to keep the command in the hands of the main combatants, France and Britain.

Sarkozy had to acquiesce to a NATO command, however, under pressure from the United States and Britain. Against that background, French officials began complaining that the campaign was bogging down within days of the U.S. withdrawal.

The unease grew as reports from Libya made it clear that rebel forces — which seemed to be winning as NATO took over — were in no shape to quickly overwhelm Gaddafi’s army and well-equipped militias.

A senior French diplomat, speaking anonymously to avoid committing the government, emphasized that France and its allies are relying on defections among Gaddafi’s aides and on diplomacy to bring the campaign to an end, realizing that the early hope of a clean rebel victory has evaporated.

Whatever the method, he said, France and its allies have decided the outcome must be Gaddafi’s departure from power.

Former Libyan foreign minister Musa Kusa, who abandoned Gaddafi late last month and fled to Britain, was set to leave Tuesday for the international meeting on Libya scheduled for Wednesday in Qatar, according to a person familiar with the matter who was not authorized to speak publicly. The trip would mark Kusa’s first since his defection, and he appeared set to offer insights on Libya to officials gathering for the forum in Doha.

U.S. officials confirmed that Musa Kusa had been invited to the Doha meeting by the host country, but said it was not clear what role, if any, the former Libyan intelligence chief would play in the meeting.

Correspondent Anthony Faiola in London and staff writer Joby Warrick in Washington contributed to this report.