The freighter hauling 850 tonnes of Japanese and German food aid was seized by unidentified pirates on Monday between Haradhere and Hobyo, about 300 kilometers northeast of Mogadishu, they said, adding that the demand for 500,000 dollars in ransom was received on Tuesday.

"They called us on the 28th and demanded a ransom of a half a million US dollars," said Karim Kudrathi of the Motaku Shipping Agency in the Kenyan port of Mombasa which owns the vessel.

"We have totally refused and are engaging our negotiators," he told AFP.

The WFP appealed for the immediate release of the ship, its 10-member crew and the food aid and urged "local authorities and community elders to intervene in this regard."

"It is against international humanitarian law to hinder the passage of humanitarian assistance and there is no justification for hijacking," the WFP said in a statement.

The Japanese- and German-donated rice on board was donated in response to a WFP appeal for assistance for some 28,000 Somalis affected by the December 26, 2004 tsunami that devastated countries around the Indian Ocean.

The ship, the St Vincent and the Grenadines-registered MV Semlow, had been on its way from Mombasa to Bossaso in Somalia's northeast Puntland region when it fell afoul of the pirates in waters deemed highly unsafe by international maritime agencies.

Both the International Maritime Board (IMB), a division of the International Chamber of Commerce, and the United States have in recent months issued increasingly dire alerts about threats to shipping off the Somali coast.