AJDABIYA, Libya — Rebel fighters claimed NATO airstrikes blasted their forces today in another apparent mistake that sharply escalated anger about the military alliance’s efforts to cripple Libyan forces. At least five rebels were killed and more than 20 injured, a doctor said.

The attack — near the front lines outside the eastern oil port of Brega — would be the second accidental NATO strike against rebel forces in less than a week. It brought cries of outrage from fighters struggling against Moammar Gadhafi’s larger and more experienced military.

“Down, down with NATO,” one fighter shouted as dozens of rebel vehicles raced eastward from the front, toward the rebel-held city of Ajdabiya.

Later, hundreds of cars poured out of Ajdabiya toward the de facto rebel capital, Benghazi, amid fears that pro-Gadhafi forces could use the disarray among rebel units to advance along the Mediterranean coastal road that serves as the lifeline for both sides.

In Brussels, a NATO official said the alliance will look into the latest rebel claims but he had no immediate information. The official spoke on condition of anonymity under standing regulations. In a separate statement, NATO dismissed Libyan claims that British warplanes struck the country’s largest oil field, saying the attacks were carried out by government forces.

NATO last week took control over the international airstrikes that began March 19 as a U.S.-led mission. The airstrikes thwarted Gadhafi’s efforts to crush the rebellion in the North African nation he has ruled for more than four decades, but the rebels remain outnumbered and outgunned and have had difficulty pushing into government-held territory even with air support.

In Washington, Army Gen. Carter Ham — who led the initial phases of the Libyan mission before transferring command to NATO — described the conflict as a stalemate and said Gadhafi’s forces are making airstrikes more difficult by mixing into civilian areas.

A rebel commander, Ayman Abdul-Karim, said he saw airstrikes hit tanks and a rebel convoy, which included a passenger bus carrying fighters toward Brega. He and other rebels described dozens killed or wounded, but a precise casualty toll was not immediately known.

An official at Ajdabiya Hospital, Dr. Mohammad Idris, said at least five people were killed and 22 injured, including some with serious burns. Idris said other casualties were left in the field in the chaos to flee the area.

In Benghazi, opposition spokeswoman Iman Bughaigis said the death toll could be as high as 13.

The small medical facility was overwhelmed. One rebel sat in a hallway, wrapping gauze around his injured leg.

Last Friday, a NATO airstrike killed 13 rebel fighters in eastern Libya. An opposition spokesman described it as an “unfortunate accident” in the shifting battles and pledged support for the international air campaign to weaken Gadhafi’s military power.

Today, NATO said the situation where the strike occurred was “unclear and fluid with mechanized weapons traveling in all directions.”

“What remains clear is that NATO will continue to uphold the U.N. mandate and strike forces that can potentially cause harm to the civilian population of Libya,” the alliance said.

But rebel discontent with NATO appears to be growing. Opposition commanders have complained in recent days that the airstrikes were coming too slowly and lacking the precision to give the rebels a clear edge. NATO officials say that the pro-Gadhafi troops have blended into civilian areas in efforts to frustrate the alliances bombing runs.

The rebel commander Adbul-Karim said the tops of rebel vehicles were marked with yellow under advice by NATO to identify the opposition forces. But rebels use tanks and other vehicles commandeered from the Libyan army — potentially making their convoys appear similar to pro-government units from the air.

The attack occurred about 18 miles (30 kilometers) from Brega, where rebel forces have struggled to break through government lines, he said.

Rebels also have turned to the oil fields under their control as a source of money for weapons and supplies. The Liberian-flagged tanker Equator, which can transport up to 1 million barrels of oil, left the eastern port of Tobruk en route to Singapore on Wednesday, oil and shipping officials said.

But sustained attacks on the main rebel-held oil fields have crippled production. Libya claimed British jets waged the bombings. NATO, however, dismissed the accusations and blamed Gadhafi’s forces.

“We are aware that pro-Gadhafi forces have attacked this area in recent days,” said Canadian Lt. Gen. Charles Bouchard, who commands the allied operation. “To try and blame it on NATO shows how desperate this regime is.”

In the capital, Tripoli, former U.S. congressman Curt Weldon met with a senior Libyan official and said it was time for Gadhafi to step down and hand power to an interim government.

The meeting between Weldon and Libya’s prime minister, Al-Baghdadi al-Mahmoudi, was part of a private mission by the former Pennsylvania lawmaker, who has visited Libya several times as Washington rebuilt ties with Gadhafi.

Weldon said he came to Libya on the invitation of Gadhafi, but the trip has no ties with the U.S. government.

Meanwhile, a former Gadhafi loyalist, Libya’s ex-energy minister Omar Fathi bin Shatwan, has held talks with British and other European diplomats to discuss the state of Gadhafi’s regime. He told The Associated Press on Wednesday that he had fled to Malta on a fishing vessel.

In London, officials said an international group overseeing political initiatives on Libya is scheduled to hold its first meeting next Wednesday in Qatar, one of the few Arab nations contributing aircraft to the NATO mission. The so-called “contact group” includes European nations, the United States, allies from the Middle East and international organizations.

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