Badly burned survivors described scenes of carnage, with the fighters killed in a flash, as a cluster of rebel vehicles were lifted off the ground and then landed in fiery wrecks.
The deaths underscored the dangers faced by Western allies as they rely on airstrikes to push back the forces of Col. Muammar el-Qaddafi, and revealed the anxieties of the rebel leadership, which fears what would happen if the airstrikes stopped.
The rebel leaders quickly called the strike an error and blamed the inexperience of their fighters. “It’s a mistake,” said Abdul Hafidh Ghoga, the rebel’s main spokesman, speaking at a meeting of the leaders, many of whom watched with grim faces as news of the airstrike was broadcast on Al Jazeera television. “Nothing has changed.”
A survivor said he believed that the airstrike had been intended for Qaddafi forces, and that the allies had bombed the wrong target because a rebel fighter had fired a heavy machine gun into the air.
The potential for such deadly mistakes could be mounting: the Qaddafi loyalists are increasingly using equipment similar to the rebels’, including pickup trucks fitted with machine guns or rocket launchers, making it difficult for even the combatants to recognize their enemies.
A NATO spokesman in Brussels said the alliance was aware of the report and was investigating.
“NATO takes reports of civilian casualties very seriously,” the spokesman said. “But for us, exact details are hard to verify because we do not have reliable sources on the ground.”
The spokesman, who, following NATO policy, asked not to be identified, added, “If someone fires at one of our aircraft, they have the right to defend themselves.”
In recent days, American officials have said that the United States, at least, has its own sources on the ground, saying that teams of C.I.A. operatives are in Libya, in part to gather intelligence for military airstrikes.
American officials have said that the United States was reducing its military role in Libya, and the Pentagon said Saturday that there would be no more American airstrikes after this weekend. But as of late Friday, the United States was still conducting most of the airstrikes and was still fully engaged early Saturday.
The strike near Brega occurred after dark on Friday as rebels were trying to retake the city. The Qaddafi forces had positioned forward observers in the desert nearby with a view of the road, enabling their artillery crews in the city to hit the rebels as they tried to approach.
A small convoy of rebel trucks and an ambulance had entered an area of close fighting between the lines of the two sides. Two of the rebel fighters there said they had been told to search an area in Brega. One of them, Ali Abdullah Abubaker, said the rebel army leader Abdul Fattah Younes had warned them to leave heavy weapons behind.
Around nightfall, the rebels had stopped for prayers on a stretch of road between Ajdabiya and Brega. Rebels driving a Mitsubishi truck with a heavy machine gun mounted on the back pulled up near the group, and one of them started firing into the air.
“I don’t know why,” Mr. Abubaker said. “Maybe he was scared.”
Other rebels speculated that there may have been celebratory firing or that Colonel Qaddafi’s forces had infiltrated rebel lines and fired at NATO warplanes.
Seconds later, Mr. Abubaker heard the planes. “I saw something white,” he said. “There was no sound.” The white pickup truck he was in burst into flames, he said, and three of the four other men in it were killed.
“It’s a mistake,” said Mr. Abubaker, a college student, who had burns on his face and was struck by bullets in the truck that ignited in the blast.
Another fighter, Ibrahim Fahim al-Oraybey, 19, who had been riding in a pickup with a machine gun mounted on the back, said he saw a shepherd who lost both arms in the blast. Mr. Oraybey was also wounded, with burns on his face, back and shoulders. On Saturday, surgeons amputated his right leg below the knee.
An ambulance driver who arrived at the scene about an hour after the strike said he found only the blackened remains of four trucks and eight or nine bodies so badly burned and mangled by the explosion that he could not determine the exact number.
“I saw the fire, and the bodies, eight or nine bodies,” said the driver, Ahmed al-Ginashi. “They were totally burned.”
On the eastern front and in the besieged western city of Misurata, rebel fighters said Saturday that they were anxious about what they perceived as a slowdown in the airstrikes that enabled Colonel Qaddafi to hold on as his forces regroup and advance. Officials said the airstrikes slowed down last week because of bad weather.
Mohamed, a Misurata resident whose surname was being withheld for the sake of his family’s safety, said that Qaddafi forces had attacked the city again on Saturday morning with tanks and mortars, firing on the Al Jazeera neighborhood near the Mediterranean. He said that at least four people were killed, and that a tally prepared by the hospital on Friday put the death toll from a month of fighting in the city at 230.
On Saturday a Turkish relief boat arrived to evacuate some wounded patients, he said.
The battle lines in the east, just east of Brega, remained largely unchanged, with Qaddafi forces in control of the city. Although airstrikes have taken out some of the government’s tanks and heavy weapons, the militia appeared to have held back some of its military equipment in the relatively dense urban area.
Despite American plans to curtail its actions in Libya by Sunday, the American presence was robust on Friday and early Saturday. NATO reported Saturday morning that it had conducted 74 sorties on Friday. The Pentagon said that about 52 of them were flown by American aircraft.
On Saturday morning, American aircraft flew 24 sorties.
No Tomahawk missiles were fired from American ships on Friday or Saturday morning, and some of the Navy warships and submarines that had launched them in the past two weeks were expected to leave the area in the coming days. As of Friday, there were 9 American ships in the Mediterranean, down from 12 at the start of military action on March 19.
American planes will still be conducting surveillance flights, radar-jamming missions and in-air refueling for France, Britain and other allied aircraft. American warplanes and ships will also remain on standby should NATO commanders need them, officials said.
In Washington, two lawmakers, Senator John McCain, an Arizona Republican, and Senator Joseph I. Lieberman, a Connecticut independent, argued in anopinion column in The Wall Street Journal that Western forces should refocus their airstrikes on toppling Colonel Qaddafi, moving beyond the United Nationsmandate to protect Libyan civilians.
“A successful outcome in Libya requires the departure of Gadhafi as quickly as possible,” the senators wrote. “It is not in our interest for Libya to become the scene of a protracted stalemate that will destabilize and inflame the region.”
They continued, “The battlefield reversals suffered by the opposition this week, when weather conditions hampered coalition airstrikes, underscore the need for a more robust and coherent package of aid to the rebel ground forces.”
As a stalemate held in the eastern front, the capital, Tripoli, remained under a tight lockdown. A panic set off by the defection of the Qaddafi confidant Moussa Koussa eased slightly as only one other high-level official appeared to have fled in his wake. According to former government officials, guards were preventing others from leaving.
One senior official who had said he planned to travel to Egypt to pick up family members canceled his trip, telling reporters that he delayed it because of a paperwork problem.