A US warship is heading towards the Somali coast where a United Nations-chartered cargo ship was hijacked by pirates, say aid officials.
UN staff say the hijacked ship, the MV Rozen, is at anchor after delivering food aid in north-eastern Somalia.
There have been no reports of demands from the pirates who boarded the ship armed with AK-47s. It is not known if any of the 12 crew have been injured.
Piracy was rampant in Somalia, but stopped during recent Islamist rule.
Since the overthrow of the Union of Islamic Courts at the turn of the year, Somalia has been descending back into the violence and chaos seen in the previous 16 years, and this latest incident raises fears of a resumption of lawlessness on the seas as well.
A World Food Programme spokesman, Said Warsame, said the US warship was still in international waters, but was heading towards the port of Bargal in the Puntland area, where the MV Rozen has docked.
US officials in Nairobi have not confirmed the reports.
"The administration in Puntland is also involved in efforts to secure the release of the vessel and its crew," Mr Warsame told the BBC.
Puntland's Ports Minister Sa'id Mohamed Raage said three police speed boats were planning to surround the pirates.
A policeman said the boats were now in range of the ship.
The crew is composed of six Kenyans and six Sri Lankans, including the captain.
"We asked them to stop going further because our biggest concern is the safety of the 12 crew on board," Col Abdi Ali Hagafe told the BBC.
WFP says there are no demands for ransom.
"We know it has been hijacked by pirates but we do not know how many pirates there are," the WFP's Stephanie Savariaud said.
Ms Savariaud said the Rozen had delivered 1,800 tonnes of food aid to the semi-autonomous Puntland region of Somalia and was returning to its home port of Mombasa in Kenya.
The Rozen was attacked last year off a port south of Mogadishu, but dodged the pirates that time, the WFP said.
In October, Kenya jailed a group of Somali pirates after they were captured by the by the US Navy off the Somali coast.
Meanwhile five people were injured when an unidentified attacker hurled an explosive device at a stall selling fuel in the Somali capital, Mogadishu.
A police officer was among those wounded in the explosion in the north of the capital.
Witnesses said the attacker escaped in a vehicle which sped from the scene.
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