Somali pirates have threatened to blow up a hijacked South Korean oil supertanker unless a $20m ransom is paid.

South Korea has sent a destroyer to intercept the Samho Dream, which has two million barrels of crude oil and its crew of 24.

It has 21 Filipino crew members, Andrew Mwangura of the Kenya-based East Africa Seafarers’ Assistance Programme told Reuters.

The Panama-flagged MV Voc Daisy was also seized 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 that favours their operations.