A Nato airstrike hit a column of rebel tanks killing at least four fighters.

This is reportedly the third incident of friendly fire involving Allied forces in the last week and has fuelled anger among rebel forces.

The most recent attack happened outside the strategic oil port of Brega raising questions about the level of co-ordination between Nato forces and the rag-tag of rebel militias who oppose Colonel Gaddafi’s 41 year rule.

Some rebels shouted insults against Nato as they retreated, UKPA reports.

Nato did not take responsibility for the mistake but hightlighted the difficulties in operating in an area that was “unclear and fluid with mechanised weapons travelling in all directions.”

“What remains clear is that Nato will continue to uphold the UN mandate and strike forces that can potentially cause harm to the civilian population of Libya,” the Allies said in a statement.

“We are not questioning the intentions of Nato, because they should be here to help us and the civilians,” rebel leader Gen Abdelfatah Yunis told a news conference in Benghazi on Thursday. “But we would like to receive some answers regarding what happened today.”

Since beginning UN-sanctioned airstrikes against Gaddafi’s forces on March 19 the Allies have failed to loosen the dictator’s grip on Libya leading to what many analysts and top military brass are calling a “stalemate”  in the civil war.

Gaddafi’s government looking for exit strategy

US may arm anti-Gaddafi rebels