A Virgin Islands-owned chemical tanker carrying 28 North Korean crew members has been hijacked by Somali pirates off the Seychelles, the multinational naval force operating in the area has said.

The European Union naval force (Navfor) said the 22,000 tonnes tanker, the MV Theresa VIII, was hijacked on Monday in the south Somali Basin, 180 nautical miles northwest of the Seychelles.

The tanker, which was operated from Singapore, had been heading to Mombasa in Kenya, but was now heading north after being turned around by the pirates, Navfor said in a statement.

Attacks on vessels off the Seychelles have surged in recent weeks as Somali pirates extend their range to evade navies patrolling off the Horn of Africa.

The world's naval powers started deploying warships in the Gulf of Aden last year in an attempt to curb attacks by ransom-hunting
pirates in one of the world's busiest maritime trade routes.

The patrols have forced the pirates to shift their focus to the wider Indian Ocean, a huge area that is difficult to police with the naval force's limited resources.

'Ransom received'

A group of Somalis holding a Spanish trawler said on Tuesday they had received a $4m ransom to release the vessel and its 36 crew immediately.

"Technically, the ship is free. We're in the process of checking the money," Abdulle said by phone from Harardhere, northern Somalia.

He said that the group holding the trawler, the Alakrana, was in agreement over the release.

The pirates have been holding the vessel and its crew since October 2.

In addition to the $4m, the pirates had also demanded the release of two of their colleagues who were arrested at sea shortly after the Alakrana was seized and brought to Spain.

It is not known whether the pirates, who have been flown to Spain and charged with armed robbery and kidnapping, were released.

The Spanish government did not immediately comment on the claims.