Somali pirates say they have released a Saudi-owned tanker carrying two million barrels of crude oil that they hijacked in the Indian Ocean.

"All our people have now left the Sirius Star. The ship is free, the crew is free," Mohamed Said, the leader of the pirate group, told the AFP news agency on Friday.

Farah Osman, an purported associate of the pirates, told the Reuters news agency that a $3m ransom had been paid for the 330-metre tanker owned by the shipping arm of oil giant Aramco.

"Pirates holding the Saudi ship took $3 million yesterday evening and then released the ship this morning," he said.

The pirates, who boarded the tanker on November 15, had demanded at least $15m to release the vessel.

Andrew Mwangura, of the maritime group the East African Seafarers Assistance, confirmed that the vessel was on the move.

"The last batch of gunmen have disembarked from the Sirius Star. She is now steaming out to safe waters," he said.

Countries including the US, Russia and India have sent naval ships to patrol waters around the Horn of Africa as acts of piracy increasingly threaten the safe passage of maritime cargo through the Gulf Aden towards the Suez Canal.

Somali pirates are holding 17 ships and more than 250 crew members hostage, and have carried out more than 100 attacks in the last year.