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.
Nato said it had carried out a strike but could not confirm rebel vehicles were hit.
Later, government troops were reported to have re-taken Kufra in the south and a crossing on the Tunisian border.
The Wazin border post - near the Tunisian town of Dehiba - had been seized by rebels last week. Fighting is reported to have spilled into Tunisian territory, along Libya's western border
Misrata has been the scene of intense fighting because of its 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.
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 confirmed alliance aircraft had struck "a number of combat vehicles 10 miles south-east of Misrata port", in an area where a large group of pro-Gaddafi fighters had been broken up on Tuesday.
The official said Nato could not "independently verify reports that these vehicles were operated by opposition forces" but said there had been no Nato attack on any building in or around Misrata.
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.
Later on Thursday, there were reports of violent clashes in the southern city of Kufra.
One rebel told AFP that 60 cars carrying some 250 pro-Gaddafi fighters had arrived in the town and retaken it.
He said the rebels had put up "light resistance" but that there were no casualties.
Another fighter said the government troops had attacked a courthouse, from which they then flew the green Libyan flag.
The rebels' National Transitional Council said told AFP were sending reinforcements to Kufra.
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.