Correction: An earlier version of this article incorrectly described demon­strations in the rebel-held city of Benghazi as occurring on Wednesday. The demonstrations, in which protesters criticized NATO for reducing the number of airstrikes, occurred Tuesday. This version has been corrected.

Libyan military commanders loyal to Moammar Gaddafi are blunting the impact of NATO’s air campaign by hiding tanks and artillery in densely populated areas where the alliance’s fighter planes cannot easily reach them, U.S. and European diplomats said Wednesday.

The shift in tactics has meant fewer targets for NATO warplanes, fueling complaints by rebels who say the quality of air support has plummeted since the United States turned over command of Libyan operations to NATO. Opposition leaders say Gaddafi’s forces are inflicting particularly heavy casualties on civilians in the rebel-held city of Misurata, where dug-in loyalists have been operating with little interference from NATO missiles and bombs.

NATO officials in Brussels acknowledged carrying out fewer strikes around Misurata because of fears of inadvertently killing civilians in areas where the Libyan military was cheek by jowl with civilians.

“We have confirmation that in Misurata tanks are being dispersed, being hidden, humans being used as shields in order to prevent NATO sorties to identify targets,” Brig. Gen. Mark van Uhm, NATO’s chief of allied operations, said at NATO headquarters in the Belgian capital.

In Washington, Secretary of State Hillary Rodham Clinton decried the use of civilians as cover and said the tactical shift had complicated NATO’s task.

“It is difficult when you have a force such as is deployed by Gaddafi, insinuating itself into cities, using snipers on rooftops, engaging in violent, terrible behavior that puts so many lives at risk,” Clinton said at a news conference with Italian Foreign Minister Franco Frattini. But she added that NATO was “performing admirably” under the circumstances.

The complaints about weakened air power came six days after the official transfer of operational command of the air campaign to NATO. The Obama administration has pressed Western and Arab allies to assume greater responsibility for the Libyan intervention while continuing to provide reconnaissance and refueling support.

But the U.S. pullback has meant a withdrawal of U.S. planes that are particularly effective in providing close air support, such as the twin-jet A-10 Warthog and the AC-130 Spectre gunship. Western diplomats say the challenge of recognizing military targets hidden among civilian apartments and schools would be daunting regardless of who was in charge of the mission.

Carmen Romero, NATO deputy spokeswoman, said air cover for the rebels had continued unabated since the United States turned over command. “The ambition and the position of our strikes have not changed,” she said.

But in Benghazi, the mood in the streets turned angry Tuesday as demonstrators blasted NATO for reducing the number of airstrikes in recent days.

At the courthouse, a hub for revolutionary activity, about 300 people waved flags and chanted slogans, demanding airstrikes from NATO and arms for the rebels.

“I know they are busy and things in Brussels take a lot of time, but I think it is late,” said Hajer Aljahi, 27, a doctor from Misurata who is living in Benghazi. “People from Zintan and Misurata are saying that Gaddafi’s coming close. Why can’t they stop them like they did in Benghazi? We are very grateful to NATO, but Misurata and Zintan are Libyan cities, too.”

“I think they will leave us to face this monster alone,” said Eyad Al Hawaz, an engineer who attended the protest with his two young children.

The sentiments echoed earlier statements by Abdul Fattah Younis, chief of staff of the rebel military, who said at a news conference in Benghazi that “NATO has disappointed us.”

“Myself and several of my officers, we have tried to give them targets to bomb so they can aid civilians,” he said, adding that it was taking NATO about eight hours to respond with airstrikes.

The Libyan government said British warplanes had carried out an airstrike Wednesday evening against the Sarir oil field, the largest in the country, which killed three oil field guards and left other employees wounded.

“We have no doubt that this aggression against the Sarir oil field is against international law and is not covered by the Security Council resolution,” Deputy Foreign Minister Khaled Kaim said at a news conference in Tripoli.

The complaints came amid diplomatic efforts to resolve the conflict. State Department officials acknowledged that a former U.S. House member from Pennsylvania, Curt Weldon (R), was in Tripoli on what was described as private trip to explore a possible peace deal.

Chris Stevens, appointed by Clinton as a U.S. envoy to the Libyan opposition, held a second day of meetings with rebel leaders in Benghazi. The Obama administration is weighing whether to extend diplomatic recognition and perhaps military aid to the main opposition group, the Transitional National Council.

State Department officials also revealed that Gaddafi wrote a personal letter to President Obama urging him to halt NATO attacks on his country.

Gaddafi addressed the U.S. president as “our son” in a note riddled with spelling and grammatical errors.

“As you know too well, democracy and building of civil society cannot be achieved by means of missiles and aircraft, or by backing armed member of al-Qaeda in Benghazi,” the Associated Press quoted Gaddafi as saying.

Clinton dismissed Gaddafi’s appeal, saying the Libyan leader “knows what he must do.”

Cody reported from Paris. Staff writer Tara Bahrampour in Benghazi and correspondent Simon Denyer in Tripoli contributed to this report.