The Somali pirates who hijacked the Saudi oil super-tanker Sirius Star have demanded a ransom, according to a purported mastermind of the daring heist.

"Negotiators are located on board the ship and on land. Once they have agreed on the ransom, it will be taken in cash to the oil tanker," said a man identified as Farah Abd Jameh on Al-Jazeera television, who did not indicate the amount to be paid.

"We assure the safety of the ship that carries the ransom. We will mechanically count the money and we have machines that can detect fake money," the man said on an audio tape produced by the Dubai-based television network.

The message came as two more ships were seized by Somali pirates in the past 24 hours: a Greek bulk carrier and a Thai fishing boat, despite a large international naval presence in the waters off their lawless country. An Iranian cargo ship was also hijacked earlier in the week.

The East African Seafarers' Assistance Programme said the Greek vessel, with up to 25 people on board, was taken on Tuesday in the Gulf of Aden. It was the third ship seized since the weekend, when pirates captured the Sirius Star, which was the largest hijack in history.

The fishing boat, which was also seized in the Gulf of Aden, had 16 crew on board and was flying a Kiribati flag but operated out of Thailand. It made a distress call late on Monday as it was chased by pirates in two speedboats but the phone line got cut off midway, said Noel Choong, who heads the International Maritime Bureau's piracy reporting centre in Kuala Lumpur.

Andrew Mwangura, coordinator of the Mombasa-based East African Seafarers' Assistance Programme, an organisation which has been monitoring this year's explosion in piracy off Somalia, said the latest spate of brazen attacks by Somali pirates means they are sending out a message.

“The pirates are sending out a message to the world that 'we can do what we want, we can think the unthinkable, do the unexpected'," Mr Mwangura told Reuters in the Kenyan port of Mombasa.

Mr Mwangura said the Greek ship had between 23 and 25 crew members but he had no further details. It followed yesterday’s hijacking, also in the Gulf of Aden, of a Hong Kong-flagged ship carrying grain and bound for Iran.

Meanwhile Foreign Secretary David Miliband has called for the immediate release of the hijacked crew of the Sirius Star - including two Britons - after the vessel anchored off the coast of Somalia.

Mr Miliband said the problem of piracy in the region was "a grave danger to the stability in the region".

Speaking to the BBC as he arrived in Beirut for talks with the Lebanese government, Mr Miliband said: "The United Kingdom is very worried about the piracy in the Gulf of Aden and in Somalia.

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