Nato has said it “strongly regret[s] the loss of life” after a “friendly fire” attack on rebel tanks in eastern Libya which left at least four dead.
Earlier, a Nato commander had refused to apologise, saying that until Thursday’s strike, the alliance had not been aware the rebels had tanks.
UK Foreign Secretary William Hague said “mistakes do happen”, but Nato should be ready to say sorry for the deaths.
Rebel forces reacted with anger at the attack on their tanks near Ajdabiya.
However, rebel commanders stressed it would not damage relations with the allied force.
On Friday, Mr Rasmussen described the situation on the ground as “fluid”, saying: “We have seen in the past that tanks have been used by the Gaddafi regime to attack civilians.”
He went on: “This is also the reason why our aircraft target military equipment that could be used to attack civilians, but I can assure you that we do our utmost to avoid civilian casualties.”
Earlier on Friday, Rear Adm Harding said: “I’m not apologising.”
He said government tanks known to have previously targeted civilians in the western city of Misrata had been on the road on Thursday. At that point, Nato did not know that rebel troops had begun to bring out their tanks.
“Up until yesterday, we had no information that the… opposition forces were using tanks,” he told reporters in Naples, near the Nato base which is co-ordinating the Libya operation.
Mr Hague told the BBC: “I think we should say that it is deeply regrettable and I think when something like this happens, it doesn’t cost anything to apologise.
“So I think we should apologise where there is error. If people are killed who are not attacking civilians, then it is a mistake.”
The rebels hit in Thursday’s air strike had been moving a group of tanks, armoured vehicles and rocket launchers near the front line between the towns of Ajdabiya and Brega in more than 30 transporters.
One rebel commander told the BBC he had seen at least four missiles land among rebel fighters.
Rebels said four fighters died, while local doctors told the BBC at least 13 had been killed in the strike. Many more were injured.
The BBC’s Wyre Davies in Ajdabiya said there was considerable anger among rebel troops about the incident. They were asking why rebel units were hit, when they could be seen clearly advancing in a westerly direction towards the front line.
Rear Adm Harding’s comments about the incident contradicted what rebel commander Gen Abdelfatah Yunis told the BBC on Thursday evening.
He said Nato had been informed that the rebels’ tanks would be on the road, and had even been given the necessary co-ordinates. He also said Nato had apologised to the rebels about the incident, although not directly to him.
Gen Yunis called on Nato to give a “rational and convincing explanation” about what had happened, but stressed the incident would not lead to tensions with the allied force.
This is the third such incident since Nato took over air operations from a US, French and British coalition a week ago.
Last Friday, at least 13 people were reportedly killed when a coalition plane fired on a rebel convoy between Brega and Ajdabiya. In a separate incident, seven civilians died and 25 were hurt in a coalition air strike on a pro-Gaddafi convoy near Brega.
Rear Adm Harding also refused to back the view of US General Carter Ham – who led the first stage of the coalition air campaign in Libya – that the conflict appeared to have reached a stalemate with rebel forces unlikely to oust Col Gaddafi’s troops.
“If someone wants to define that as a stalemate that’s fine, all I’m saying is that yes, it’s fluid, but it’s fluid in a relatively small area,” he said.
Meanwhile, pro-Gaddafi troops are reported to be advancing into the eastern districts of Libya’s third biggest city, Misrata, triggering street battles with rebels.
The UN children’s agency, Unicef, said it had “reliable and consistent reports” that snipers had hit children in the city.
Misrata has been under attack by Libyan government forces for several weeks, and Libyan rebels had complained it would “cease to exist” within a week unless Nato took action to save it.
A ship chartered by the UN World Food Programme has delivered hundreds of tonnes of high-energy biscuits, flour, and water purification tablets, as well as enough medicine to last 30,000 people for a month.
Source: Joy News/Ghana