Libyan rebels have accused the forces of Libyan strongman Muammar Gaddafi of using helicopters bearing the Red Cross emblem to drop mines into the harbour of the besieged city Misrata.

NATO confirmed that helicopters had flown over the city on Thursday in breach of the no-fly zone its war planes are supposed to enforce, but it could not confirm that the choppers were marked with the Red Cross sign.

Suleiman Fortiya, who represents Misrata on the Benghazi-based opposition National Transitional Council said small helicopters flew over Misrata on Thursday and yesterday to drop mines in the rebels' major western hold-out.

He said the choppers had been disguised as humanitarian aircraft carrying the emblems of the International Committee of the Red Cross and Red Crescent.

"They had Red Crescent and Red Cross markers so that anyone who sees them thinks it is for humanitarian aid," Fortiya said.

An aid worker said he saw helicopters yesterday marked with the Red Crescent circling above the port and dropping mines into the sea.

A NATO official told AFP a ship involved in the the coalitions' operations had observed a number of helicopters over Misrata on Thursday.

"We are aware of reports that the helicopters were marked with the Red Cross," said the NATO official, adding that no humanitarian flights had been notified for the Misrata area on that day.

"Any use of the Red Cross to disguise combat forces would be a breach of international law," he said.

A spokeswoman at the ICRC's Geneva headquarters said they had received similar reports but could not confirm them, as the organisation currently has no team on the ground.