Somali pirates who seized a Japanese chemical tanker six weeks ago have demanded a $1m ransom.
Piracy is a major problem for shipping off Somalia
They have threatened to kill the 23-man crew unless the ransom is paid.
The tanker, the Golden Nori, was sailing from Singapore to Israel with a cargo of chemicals when it was attacked by pirates at the end of October.
Her distress calls were picked up by US warships patrolling off the coast of Somalia who fired on the pirates' speedboats, sinking two.
The warships followed the kidnapped chemical tanker into Somali coastal waters and have trapped the Golden Nori close to the Somali port of Bossaso, preventing supplies from getting to the ship.
The BBC's East Africa correspondent Adam Mynott says Somali coastal waters are now considered to be among the most hazardous in the world.
With no effective government, pirates are able to operate largely unchecked, our correspondent says.