A Nato air strike intended to thwart Muammar Gaddafi’s forces killed 13 rebel fighters in eastern Libya instead, the opposition said today.

Government forces killed six civilians in the city of Misrata today in an unrelenting campaign of shelling and sniper fire aimed at driving rebels from the main city they hold in western Libya, medical officials said.

Nato is investigating reports from Libyan rebels that a coalition warplane struck one of their positions that was firing into the air near the front line of the battle with Muammar Gaddafi’s forces.