The Saudi authorities have called the hijacking an outrageous act, with the foreign minister saying the kingdom would now throw its weight behind an international crackdown on piracy.
Prince Saud al-Faisal, the foreign minister, said on Tuesday Saudi Arabia would back a European-led initiative to step up security in shipping lanes off Africa's east coast.
"This is an initiative that we are going to join and so are many other countries of the Red Sea," he told a news conference in Athens.
"This outrageous act by the pirates, I think, will only reinforce the resolve of the countries of the Red Sea and internationally to fight piracy."
The vessel is owned by Saudi oil giant Aramco and was fully loaded when it was attacked more than 450 nautical miles southeast of Mombasa.
"All 25 crew members on board are believed to be safe," Vela International, the shipping arm of Saudi Aramco, said in a statement.
"At this time, Vela is awaiting further contact from the pirates in control of the vessel."
|Saudi foreign minister Saud al-Faisal said the hijacking was an outrageous act [AFP]
Richard Mitchelson, a former special forces soldier and security consultant based in Singapore, told Al Jazeera that the pirates have refined their technique over time.
"They have a lot of larger vessels," he said.
"They use a mother craft so they can position themselves in open water, then they use smaller, faster speedboats and use caving ladders to gain access to the decks [of the vessels they target]. Once they are on, it is pretty easy. They are well armed.
"They have access to GPS [global positioning system] and satellite phones ... I'm sure they would have some form of intelligence as to which vessels are coming their way."
The pirates have driven up insurance costs, forced some ships to go round South Africa instead of through the Suez Canal and secured millions of dollars in ransoms.
The capture of the Sirius Star is the culmination of several years' increasing activity.
"The latest attack looks like a deliberate two fingers from some very bright Somalis. Anyone who describes them as a bunch of camel herders needs to think again," a Nairobi-based Somalia specialist said.
The seizure was carried out despite an international naval response, including from the Nato alliance and European Union, to protect one of the world's busiest shipping areas.
"The world has never seen anything like this ... The Somali pirates have hit the jackpot"
Andrew Mwangura, co-ordinator of the East African Seafarers' Association
US, French and Russian warships are also off Somalia.
Andrew Mwangura, co-ordinator of the East African Seafarers' Association, said: "The world has never seen anything like this ... The Somali pirates have hit the jackpot."
The association, based in the Kenyan port city of Mombasa, has been monitoring piracy for years.
Mwangura said he thought a hijacked Nigerian tug was a "mother-ship" for the November 15 seizure.
"The supertanker was fully loaded, so it was probably low in the water and not that difficult to board," he said, adding that the pirates probably used a ladder or hooked a rope to the side.
The Sirius Star, weighing 318,000 tonnes and flagged in Liberia, is operated by Dubai-based Vela International.
The crew members come from Croatia, Britain, the Philippines, Poland and Saudi Arabia.
The tanker can hold up to two million barrels of oil - more than one quarter of Saudi Arabia's daily exports.
It had been heading for the US via the Cape of Good Hope at the southern tip of Africa, instead of heading through the Gulf of Aden and the Suez Canal.
Meanwhile, the official Chinese news agency, Xinhua, reported on Tuesady that a Hong Kong cargo ship Delight had been hijacked by pirates in the Gulf of Aden near the Yemen coast.
Xinhua said the ship, with 25 crew members, was carrying 33,000 tonnes of wheat and was heading for the Bandar Abbas port of Iran.
Last week, the European Union started a security operation off the coast of Somalia to combat growing piracy and protect ships carrying aid agency deliveries.
It is the EU's first-ever naval mission.
Pirates are well organised in the Horn of Africa, the area where Somalia's northeastern tip juts into the Indian Ocean.
Somalia has had no effective government since the 1991 overthrow of Mohamed Siad Barre, the former president, touched off a bloody power struggle that has defied numerous attempts to restore stability.