Seven civilians died and 25 were hurt in a coalition air strike on a pro-Gaddafi convoy in eastern Libya, a doctor there has told the BBC.
Dr Suleiman Refardi said Wednesday's raid happened in the village of Zawia el Argobe, 15km (9 miles) from Brega.
The strike hit a truck carrying ammunition, and the resulting explosion destroyed two nearby homes.
All the dead were between the ages of 12 and 20, Dr Refardi said. Nato says it is investigating the claim.
The news comes as the chairman of the rebel Transitional National Council, Mustafa Abdul Jalil, said it would agree to a ceasefire if Col Muammar Gaddafi's troops withdrew from cities.
"We agree on a ceasefire on the condition that our brothers in the western cities have freedom of expression and also that the forces that are besieging the cities withdraw," he told a news conference in the eastern rebel stronghold of Benghazi.
But he said the rebels would not back down on their demand that Col Gaddafi must go.
Libyan government spokesman Moussa Ibrahim dismissed the offer by its former justice minister, telling reporters: "The rebels never offered peace. They don't offer peace. They are making impossible demands."
"We will not leave our cities. We are the government, not them."
Dr Refardi told the BBC that the Libyan government convoy had included tanks, artillery and trucks carrying ammunition.
A direct hit on an ammunition truck and trailer in a street in Zawia el Argobe sent a hail of shrapnel into nearby houses, he said.
Four of the dead were female, including three children from the same family, aged between 12 and 16, the BBC's Ben Brown reports from Brega.
Three boys, aged between 14 and 20, were also killed.
Dr Refardi said he had spoken to the family of the girls who had been killed and "there was no anger" at the coalition forces.
"If these tanks had entered Ajdabiya it would have been a massacre," he said.
"They [the Libyan people] are expecting more than this, because they know the Gaddafi forces are using civilians as a shield."
Nato officials told the BBC they were making inquiries "down our operations chain to find out if indeed there is any information on the operation side that would support this claim".
Later, Libyan government spokesman Moussa Ibrahim said six civilians had been killed by the "immoral" air strike, and described it as a "crime against humanity".
"Some mad and criminal prime ministers and presidents of Europe are leading a crusade against an Arab Muslim nation," he told a news conference in Tripoli. "Sounds familiar? It's a crime against humanity."
Meanwhile, the BBC's Wyre Davies in Ajdabiya says rebel forces there are pressing on to the front line around the oil town of Brega.
Plenty of enthusiastic, if disorganised, insurgents are driving through in pick-up trucks with heavy-duty machine-guns mounted on the back, he reports.
The BBC's Nick Springate in Brega says the flagging morale of rebel fighters - in general poorly equipped and untrained - has been boosted by a number of well-armed, apparently well-trained soldiers in full military uniform. It is not clear where they have come from, our correspondent adds.
Gen Abdel-Fatah Younis, the former Libyan interior minister who defected to the rebels, has also addressed rebels outside Brega.
Earlier, the BBC learned that a Libyan government envoy, Mohammed Ismail, had been in London in the past few days for talks with UK authorities.
The UK Foreign Office says that in all its contacts with Libyan officials, it had made it clear that "Gaddafi has to go".
News of the visit emerged after former Libyan Foreign Minister Moussa Koussa flew to Britain and was said to have resigned his post.
There have since been other unconfirmed reports that more senior Libyans are defecting.
Al-Jazeera TV said the intelligence minister, deputy foreign minister and General People's Congress speaker were awaiting flights in Tunisia.
Oil minister Shukri Ghanim has denied reports he is to leave Libya.
He told the BBC's Today radio programme: "I am in Tripoli working in my office. I am trying my best to keep this oil industry as one industry, trying to minimise the damage that is happening to the installation, trying to secure the safety of the staff and the personnel [and] trying to prevent the looting of the industry."