At least 11 rebel fighters have been killed in a Nato air strike in the besieged Libyan port city of Misrata, say reports.
A rebel commander and witnesses told reporters a Nato warplane had carried out Wednesday's bombing, but Nato refused to confirm or deny the reports.
A doctor told Reuters news agency seven rebels had also been killed in fire from government forces.
There has been intense fighting over Misrata's strategically crucial port.
Rebel fighters, backed by Nato air strikes, claim they have driven back soldiers loyal to Libyan leader Col Muammar Gaddafi.
The port is a vital lifeline, permitting aid deliveries and refugee evacuations, but they have been interrupted by the fighting - prompting residents to warn that supplies of food and water are dwindling.
For some two months, Misrata has been besieged by Col Gaddafi's forces, and rights groups say hundreds of civilians have been killed in the crossfire.
Rebels desperate to hold the city have appealed for Nato to step up its air strikes, but reports suggest a strike on Wednesday afternoon went astray with fatal consequences.
Survivors quoted by US broadcaster CNN said 11 fighters had been killed and two injured in the strike on the coast, while a rebel commander, Abdullah Mohammed, told the New York Times 12 had died and five had been injured in apparently the same attack.
Mr Mohammed said rebels had at first been reluctant to confirm Nato's deadly mistake, out of fear it would discourage Nato from mounting further strikes.
He said Wednesday's strike was an accident that could have been avoided, but added: "We hope this does not delay strikes on our enemy."
A Nato official who spoke on condition of anonymity told the BBC it was aware of the reports and was looking into them but at this stage could not confirm or deny the strike.
Earlier this month several rebel fighters died in a mistaken Nato air raid on the eastern city of Ajdabiya.
Meanwhile, an unnamed doctor told Reuters seven rebel fighters had been killed and four injured when they were hit by artillery fire and rockets from pro-Gaddafi forces at a checkpoint near the front line.
Injured Libyans were among those hastily evacuated out of Misrata when an aid ship was able to dock In recent days fighting over Misrata port has intensified and rebels say on Tuesday pro-Gaddafi forces fired Russian-made Grad rockets, which rights groups say should not be used in civilian areas.
The Libyan government denies indiscriminately shelling civilian areas.
The EU commissioner for humanitarian relief, Kristalina Georgieva, warned she had received reports "of hospitals being overwhelmed by a growing number of wounded" and appealed for "all sides in this conflict to protect civilians and to allow humanitarian operations in Misrata to resume".
She said fighting near the port had interrupted aid supplies and made it near impossible for civilians and the wounded to be evacuated.
A resident told AP news agency food stocks were dwindling and there was a shortage of drinking water.
But an aid ship, the Red Star, was able to take advantage of a brief lull in the fighting on Wednesday to dock and pick up Libyans and stranded migrant workers.
"Despite heavy shelling of the port area... about 935 migrants and Libyans have been rescued and are now safely en route to [the rebel stronghold of] Benghazi," the International Organisation for Migration (IOM) said.
The fighting was continuing on Thursday, with some rebels quoted as saying they had made significant strides against loyalist forces.