Ecoterra International said the MT Aspahalt Venture was seized in the Indian Ocean early Wednesday morning on its way to the Kenyan port of Mombasa.
"Information ... says a pirate group from Brava had captured the vessel and is heading towards Harardheere at the Central Somali Indian Ocean coast," the group said in a statement.
Piracy is rife off the Horn of Africa nation, which has not had a functioning central government since 1991.