Andrew Mwangura, head of the Kenyan chapter of the Seafarers Assistance Programme, said the pirates had not yet made any demands.
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.
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The ship is currently being held close to the island of Ras Afun, just off the Puntland coast.
The vessel has lost contact with its home port of Mombasa in Kenya and it is unclear if the hijackers are armed, said Mwangura.
Piracy is common off the coast of Somalia, which has no effective government of its own to respond.
There were 35 instances of piracy off the coast of Somalia in 2005, compared to two in 2004, according to the International Maritime Bureau.