Somali pirates have seized three more ships since the weekend's spectacular capture of a Saudi supertanker carrying $100 million of oil that was the largest hijack in history.

The latest ships to be seized are one from Hong Kong, another one from Greece and a Thai fishing ship registered in Kiribati.

The wave of Somali piracy this year has driven up insurance costs, made some shipping companies change their routes and prompted an unprecedented military response from Nato and the European Union among others.

"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'," Andrew Mwangura, coordinator of the East African Seafarers' Assistance Programme, told Reuters in Mombasa.

His group, which monitors attacks at sea, said the Greek ship was taken on Tuesday in the Gulf of Aden with about 25 crew on board. He had no further details but it followed the hijacking of a Hong Kong-flagged ship carrying grain and bound for Iran.

An explosion in piracy this year off the coast of the poor and chaotic Horn of Africa country has been fuelled by a growing Islamist insurgency onshore and motivated by the lure of multi-million-dollar ransoms.

No ransom has been demanded so far for the Saudi supertanker Sirius Star, which the pirates seized on Saturday after dodging international naval patrols in their boldest strike yet.

A spokesman for the owners, Saudi Aramco, said the company hoped to hear from the hijackers later on Wednesday.

The hijacking took place 450 nautical miles southeast of Mombasa, far beyond the gangs' usual area of operations. On Wednesday, it was believed to be anchored near Eyl, a former Somali fishing village now used as a well-defended pirate base.

"Eyl residents told me they could see the lights of a big ship far out at sea that seems to be the tanker," Aweys Ali, chairman of Somalia's Galkayo region, told Reuters by telephone.

Somali gunmen were believed to be holding about a dozen ships in the area, and more than 200 hostages. Among the vessels is a Ukrainian ship loaded with 33 tanks and other weapons that was captured in another high-profile strike earlier this year.

The seizure of the Sirius Star was carried out despite an international naval response, including from Nato, to guard one of the world's busiest shipping routes. Warships from the United States, France and Russia are also off Somalia.

Given that the pirates were well-armed with grenades, heavy machineguns and rocket-launchers, the foreign forces were steering clear of direct confrontation, and in most cases the owners of the hijacked ships were trying to negotiate ransoms.

British Royal Navy Commodore Keith Winstanley, deputy commander of the Combined Maritime Forces in the Middle East, said  coalition forces could not be everywhere.

"The pirates will go somewhere we are not," he told Fairplay, part of defence analysts Jane's Information Group. "If we patrol the Gulf of Aden then they will go to Mogadishu. If we go to Mogadishu, they will go to the Gulf of Aden."    
In a show of resolve, Kenyan police paraded eight suspected pirates in a Mombasa court on Wednesday. The Royal Navy captured them, and killed two others, in the Gulf of Aden last week.

Also on Wednesday, South Korea said it was planning to send navy ships to the waters off Somalia to protect commercial vessels from pirates, and Japan was considering a similar move. - Reuters