A Western coalition air strike hit a group of Libya's opposition fighters on the eastern outskirts of Brega late on Friday, killing at least 13 of them,
an opposition spokesman said late on Saturday.
"Some of Gaddafi's forces sneaked in among the rebels and fired anti-aircraft guns in the air," opposition fighter Mustafa Ali Omar had said earlier. "After that the NATO forces came and bombed them."
Opposition fighters at the scene had said as many as 14 people may have died in the bombing, which they said happened around 10 p.m. local time (2000 GMT)
Most blamed a Gaddafi agent for deliberately drawing the friendly fire but some said other fighters had shot into the air by accident.
"The revolutionaries shot up in the air and the alliance came and bombed them. We are the ones who made the mistake," said one fighter who did not give his name.
Meanwhile, fighting raged on the front line between opposition-held eastern Libya and the loyalist-held west as opposition fighters claimed to have captured the strategic oil town of Brega.
Fighting appeared to have subsided on Saturday morning on the outskirts of the town and only the sound of NATO fighters flying overhead disturbed the calm.
Brega, 800 kilometers (500 miles) east of Tripoli, has been the scene of intense exchanges over the past few days when pro-Gaddafi forces returned after being driven out by the opposition fighters.
But it has been unclear since Thursday who actually held the town with the opposition forces regrouping in Ajdabiya, 80 kilometers (50 miles) to the east.
In the western part of the country, fighting flared around the opposition-held city of Misrata and air strikes were reported elsewhere in the country after Gaddafi’s regime rejected an opposition offer of a ceasefire.
Ready for truce
In the opposition bastion of Benghazi, Transitional National Council leader Mustafa Abdul Jalil said the opposition was ready for a truce, on condition that Gaddafi left Libya and his forces quit cities now under government control.
"They are asking us to withdraw from our own cities .... If this is not mad then I don't know what this is. We will not leave our cities," government spokesman Mussa Ibrahim told reporters in Tripoli a few hours later.
The U.S. military was poised to withdraw its combat jets and Tomahawk cruise missiles from the air campaign against Libya's regime starting this weekend, as NATO allies take the lead in bombing Gaddafi’s forces.
The move follows pledges by Obama to quickly shift command of the operation to NATO, with the U.S. military playing a supporting role -- providing planes for mid-air refueling, jamming and surveillance.
Call for heavy armaments
Opposition commanders called for more air strikes by coalition forces enforcing a U.N.-mandated no-fly zone over Libya, but the U.S. military's top officer said bad weather was hampering the air campaign.
Without air support, the ill-equipped opposition forces were pushed back 200 kilometers (125 miles) from the key oil hub of Ras Lanuf on Wednesday all the way east of Brega, where they regrouped on Thursday for the counter-offensive.
But the opposition calls for heavy armaments to match the superior firepower of Gaddafi’s army have been greeted with little enthusiasm by western powers.
U.S. Defense Secretary Robert Gates asserted the rebels needed training more than guns but suggested other nations do that job.
His French counterpart Gerard Longuet said providing weapons was not part of the U.N. mandate, and NATO chief Anders Fogh Rasmussen also ruled out such a move.
"We are there to protect the Libyan people, not to arm people," Rasmussen said.
The rebels said they have signed a deal with Qatar to market their crude oil abroad in exchange for food, medicine and -- they hope -- weapons.