South Korea has sent a destroyer to intercept the Samho Dream that was hijacked by Somali pirates with two million barrels of crude oil and 24 crew on board.
The naval ship was on patrol in the Gulf of Aden before diverting on Friday 930 miles south-east to the area where the hijacking occurred.
The pirates have threatened to blow up the South Korea-operated, Singapore-owned oil supertanker unless a $20m ransom is paid.
The ship was on its way to Iraq from the US.
The Panama-flagged MV Voc Daisy was seized, too, in the early hours of Wednesday, 190 miles south-east of the Omani port of Salalah.
Maritime experts say the pirates have stepped up attacks, largely due to good weather, which favours their operations.