NAIROBI, Kenya - Pirates hijacked a cargo ship delivering UN food aid to northeastern Somalia on Sunday -- at least the second time in recent years that a vessel contracted to the United Nations has been hijacked off the country's dangerous coast.

The ship, MV Rozen, had just dropped off more than 1,800 tons of food aid in the semiautonomous region of Puntland in northeastern Somalia when the pirates struck, said Stephanie Savariaud, a spokeswoman for the UN's World Food Program.

It was not immediately known if any of the 12 crew members aboard -- six from Sri Lanka and six from Kenya -- were injured in the attack.

"We know it has been hijacked by pirates but we do not know how many pirates there are," Savariaud said. "We are very concerned about the safety of the crew."

There was an attempted hijacking on the same ship in March last year by five pirates armed with machine guns and rocket-propelled grenades, but the vessel managed to outrun them.

"The pirates have not made any demands yet," said Andrew Mwangura, head of the Kenyan chapter of the Seafarers Assistance Program.

The ship was contracted by the WFP to deliver food aid to Somalia, where around one million people are suffering from a drought that hit the region last year.

The UN has explored alternative transport routes to Somalia. But overland routes are also troubled by the lack of security and lawlessness -- aid workers have been the targets of kidnappings and killings.

The WFP ship is currently being held close to the island of Ras Afun, just off the Puntland coast. The ship has lost contact with its home port of Mombasa in Kenya, said Mwangura.

The roughly 3,000 kilometre coast of Somalia, which has had no effective government since warlords ousted a dictatorship in 1991 and then turned on each other, has emerged as one of the most dangerous areas for ships.

Somali pirates are trained fighters, often dressed in military fatigues, using speedboats equipped with satellite phones and Global Positioning System equipment. They are typically armed with automatic weapons, anti-tank rocket launchers and various types of grenades, according to the U.N. Monitoring Group on Somalia.

In 2005 pirates overwhelmed one vessel carrying UN World Food Program aid, and the number of overall at-sea hijackings was 35, compared with two in 2004, according to the International Maritime Bureau.

The bandits target both passenger and cargo vessels for ransom or loot, using the money to buy weapons.