Somali pirates hijacked a Singapore-owned product tanker about 180 miles off the coast of Kenya, said port officials and the EU's anti-piracy taskforce on Monday.
The 21,000-tonne vessel MV Gemini, operated by Singapore-based Glory Ship Management, was seized on Saturday while on its way to Kenya from Malaysia.
The ship had 25 crew members on board from South Korea, Indonesia, China and Myanmar, the EU Navfor said.
"It is likely to be taken back to the Somali coastline where it will be held for a ransom, although it may also be used as a mothership in order to assist attacks on other vessels," said port and logistics firm GAC.
Pirates from the lawless Horn of Africa nation continue to outwit an international flotilla of warships patrolling the busy trade routes.
Typically the pirates anchor the vessels off their land base until a ransom is paid, at which point the vessel along with its crew and cargo is released.
More than 40 % of the world's seaborne oil supply passes through the Gulf of Aden and the Arabian Sea and is at risk from pirate gangs.