Somali pirates have seized another ship, a Greek bulk carrier, despite a large international naval presence in the waters off their lawless country, a regional maritime group said on Wednesday.

 The East African Seafarers' Assistance Programme said the Greek vessel was taken on Tuesday in the Gulf of Aden, the second ship seized since the weekend's spectacular capture of a Saudi supertanker that was the largest hijack in history.

"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 Mombasa-based group, told Reuters in the Kenyan port of Mombasa.

His organisation has been monitoring this year's explosion in piracy off Somalia, fuelled by an Islamist insurgency onshore and motivated by the lure of multi-million-dollar ransoms.

No ransom has been demanded so far for the Saudi-owned supertanker Sirius Star, a ship three times the size of an aircraft carrier and loaded with oil worth $100 million.

In their boldest and furthest strike yet, pirates dodged international naval patrols to seize it 450 nautical miles southeast of Mombasa on Saturday, far beyond their usual zone of operations. They brought it back to Somalia, where it was believed to be anchored near the port of Haradheere.

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

Rampant piracy in the region has driven up insurance costs, made some shipping companies take a route round South Africa instead of through the Suez Canal, and prompted an unprecedented response from NATO and the European Union among others.

Somali pirates are holding about a dozen ships and more than 200 hostages, most near Eyl village on the northern coast. - Reuters