The super-tanker, carrying 2m barrels of oil and a crew of 25, was hijacked 450 nautical miles east of Kenya on Sunday.
The 318,000 tonne VLCC is the largest vessel seized in an epidemic of piracy in the region.
Operating company Vela International said all crew members are safe aboard the ship. The company said the tanker was loaded at its full capacity with oil valued at $100m (€79.2m).
Sirius Star, which is owned by the Saudi oil company Aramco, is carrying crew members from Croatia, Britain, Philippines, Poland and Saudi Arabia, according to a US Navy statement.
International security fears have been further heightened after a Hong Kong cargo ship was hijacked this afternoon in the Gulf of Aden near the Yemen coast.
The China Maritime Search and Rescue Centre said the freighter, The Delight, is carrying 25 crew and 36,000 tonnes of wheat.
Piracy
The International Maritime Bureau has reported that at least 84 ships have been attacked off Somalia since January, of which 34 were hijacked.
Of those, 13 vessels and more than 200 crew are still in the hands of pirates.
A chemical tanker, the Stolt Valor, was released on Sunday payment of a $1.1m ransom.
Pirates have taken three other ships within the past week alone; another chemical tanker chartered by chemical tanker shipping group Stolt-Nielsen, a Chinese fishing vessel and a Turkish tanker.
Japanese cargo ship Chemstar Venus was hijacked late on Saturday with 23 crew.
Pirates are well organised in the area where Somalia's northeastern tip juts into the Indian Ocean, preying on a key maritime route leading to the Suez Canal through which an estimated 30% of the world's oil transits.
They have driven up insurance costs, forced some firms to go round South Africa instead of through the canal, and secured millions of dollars in ransom.
