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

Rebels are holding out in part of the strategic oil port of Ras Lanouf after fierce fighting with Muammar Gaddafi loyalists in central Libya.

Hundreds of Libyan rebels have fled a key oil port under a barrage of rockets and tank shells from government forces.

Forces loyal to Muammar Gaddafi, some in helicopter gunships, pounded opposition fighters with artillery, rockets and gunfire, dramatically escalating their counter-offensive to halt the rebels’ rapid advance towards the capital.

Hundreds of armed anti-government forces backed by military defectors who control the city closest to the capital Tripoli today prepared to repel an expected offensive by forces loyal to Muammar Gaddafi surrounding Zawiya.