An aid ship has been attacked by forces loyal to Muammar Gaddafi while rescuing African and Asian migrant workers from the besieged port of Misurata, forcing it to leave behind hundreds of Libyans desperate to flee the fighting.

Aid workers had earlier scrambled to embark the migrants, along with journalists and the wounded, on the ship bound for rebel-held Benghazi as the port came under bombardment on Wednesday. 

"The bombing has caused so many casualties among Libyans and people of other nationalities waiting for evacuation," Gemal Salem, a rebel spokesman, told Reuters news agency.

"So far we have five killed and ambulances are rushing to the scene."

The MV Red Star One, sent by the International Organization for Migration (IOM), picked up 800 people caught up in the civil war who had been waiting for days to escape Misurata's worsening humanitarian crisis.

It had hoped to take 1,000 people.

Humanitarian concern

"Hundreds of Libyan civilians had also tried to board the ship in desperation to get out of Misurata. But with a limited capacity, the ramp of the boat had to be pulled up so that the ship could pull away from the dock in safety," the IOM said.

The port is a lifeline for Misurata, where food and medical supplies are low and snipers shoot from rooftops.

Other rescue ships are offshore but there was no news of their movements. About 12,000 people have so far been rescued by 12 ships.

The shelling also hit Misurata's Qasr Ahmad district, a mixed residential and industrial area which houses the iron and steel works in a city that has become one of the bloodiest battlefields in the two-month conflict.