A Thai fishing boat taken hostage in the Gulf of Aden is now one of three vessels that have reportedly been hijacked off the coast of Somalia this week.

This undated picture taken at an unknown location shows the MV Sirius Star, a huge Saudi oil tanker that has been hijacked by Somali pirates. (Christian Duys/Associated Press)

The vessel was seized Monday as it travelled toward the Middle East, said Noel Choong, who heads the International Maritime Bureau's piracy reporting centre in Kuala Lumpur. He said there are 16 crew members on board.

The Thai boat was flying the flag of Kiribati, an island nation off the coast of Australia, but operated out of Thailand, Choong said Wednesday morning.

The news came several hours after pirates who hijacked a Saudi-owned supertanker anchored the vessel off the north coast of Somalia Tuesday. Another cargo ship was also seized in the area. Choong said a total of 17 vessels are currently being held hostage in Somali waters.

The Saudi tanker, the Sirius Star, was anchored near Harardhere, some 425 kilometres from Eyl, according to witnesses from nearby impoverished Somali fishing villages.

Abdinur Haji, a fisherman in Harardhere, told the Associated Press that he saw two small boats float out to the ship. He said 18 men, believed to be pirates, climbed aboard with a rope ladder.

Hours later, the U.S. navy said a ship, flying a Hong Kong flag but operated by the Islamic Republic of Iran Shipping Lines, was attacked Tuesday in the Gulf of Aden.

Reuters, quoting China's official Xinhua news agency, said the cargo ship was loaded with 36,000 tonnes of wheat and bound for Iran.

U.S. navy Cmdr. Jane Campbell of the 5th Fleet said the status of the crew and cargo was not known.

Rescue plan ruled out

Meanwhile, the U.S. and other naval forces have decided against rescuing the Sirius Star, which is loaded with two million barrels of crude oil valued at around $100 million. The ship, with 25 crew members on board, was seized over the weekend by Somali pirates, some 830 kilometres off the Kenyan coast.

NATO said it would not divert any of its three warships from the Gulf of Aden, and the U.S. Navy's 5th Fleet also said it did not expect to send ships.

The Dubai-based owner of the Saudi tanker Vela International Marine Ltd. said the oil tanker's crew "are believed to be safe." The crew consists of 19 Philippine nationals, two British, two Polish, one Croatian and one Saudi national.

Abdullkadir Musa, the deputy seaport minister in northern Somalia's breakaway Puntland region, said his forces will rescue the ship if it tries to anchor anywhere near Eyl.

But Musa's threat to take action was met with skepticism by observers of the largely lawless Somalia, which has not had a functioning government since the early 1990s, the CBC's Adrienne Arsenault reported.

Pirate ransoms 'filter out' into Somali society: analyst

Analysts say the pirate actions are extensions of abductions for profit on land in Somalia, with greater returns.

The pirates have already netted an estimated $20 million from ransoms this year alone, and are popular and protected on land, said Roger Middleton, an East Africa specialist at the Chatham House think-tank in London.

"When they are holding a ship off the coast, they need to buy food, water for that ship, and they buy it from the local villages, creating a kind of boom in these economies," he said.

"They build themselves big houses and so on, so the money is sort of filtering out into Somali society."

With files from the Associated Press