Kenya ? Somali authorities have arrested six suspected pirates in the hijacking of a U.N.-chartered cargo ship delivering food aid, officials said Tuesday.

Four heavily armed pirates still had control of the vessel and were holding 12 crew members hostage, said the U.N food agency. The ship, the MV Rozen, had been contracted to deliver aid to Somalia, where around 1 million people are suffering from a drought that hit the region last year.

Four suspects were seized after they went ashore to buy supplies, Peter Goossens, the head of the World Food Program in Somalia, said in a statement. Sa’id Mohamed Raage, the regional fishing minister, said police arrested two others separately.

The pirates are armed with AK-47 assault rifles and rocket-propelled grenade launchers, said Andrew Mwangura, head of the Kenyan chapter of the Seafarers Assistance Program, an independent group that monitors piracy in the region.

“Negotiations are under way to try and secure the release of the vessel,” he added. The condition of the six Sri Lankan and six Kenyan crew members was unknown.