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MONROVIA - Pirates armed with machetes boarded a broken-down cargo ship off Liberia's capital Monrovia, beat up the crew and later towed it away towards Ivory Coast, the vessel's Bulgarian owner said today.

The 67-metre MV Tahoma Reefer had been docked off Monrovia after a fire destroyed its upper deck last August, owner Valentine Mihakov said. He said he had planned to tow the ship to Italy for repairs and then later sell it on.

"The ship was stolen by pirates who came on two fishing boats ... they beat up my crew and three of them are in hospital," Mihakov told Reuters by phone from Spain.

"I want to know what has happened because I just bought the boat two months ago."

The captain of the ship, Volodymr Shteynberh, said the vessel had been stolen while his crew were in hospital being treated for wounds received in the initial attack.

He said he had informed United Nations' police officers, in Liberia to restore order after an on-off 1989-2003 civil war, as well as British diplomats in the West African country.

The freighter is registered to St Vincent and the Grenadines, a tiny Caribbean state which won independence from Britain in 1979.

A helicopter from the UN peacekeeping mission in Liberia (UNMIL) spotted the freighter being towed towards Ivorian waters but was not able to intercept it, UN spokesman Ben Malor said.

"We got a report that the vessel had gone missing. An UNMIL helicopter deployed on a search operation and spotted it being towed by two small boats some 13 nautical miles offshore towards Ivory Coast," he told Reuters.

"But UNMIL does not have the maritime capacity to get the vessel. The best we can do is monitor the situation and support the authorities in Liberia," he added.

Mihakov said his local contacts had told him the French navy, based off Ivory Coast, were involved in the search for the vessel. French military officials in Ivory Coast could not immediately confirm those reports.