The Irene SL, the length of three soccer pitches and with 25 crew members on board, was carrying about 2 million barrels of oil, or nearly one fifth of daily U.S. crude imports.
The hijacking came a day after an Italian tanker carrying oil worth more than $60 million was snatched by Somali pirates, reinforcing industry fears that the piracy scourge is "spinning out of control".
"This morning the vessel was attacked by armed men," the Irene SL's Greece-based manager Enesel said. "For the moment there is no communication with the vessel."
Commander Susie Thomson, spokeswoman for the multinational Combined Maritime Forces fighting piracy in the area, said the 333-metre tanker was hijacked 220 miles off Oman and was likely to have been attacked by Somali pirates. "We can only speculate as to where the ship is being taken."
Shipping industry associations have warned that over 40 percent of the world's seaborne oil supply passing through the Gulf of Aden and the Arabian Sea is at risk from Somali pirates, who are able to operate ever further out to sea and for longer periods, using mother ships.
"This is a strategic area of concern because it implicates shipping travelling to and from the Persian/Arabian Gulf," he said. "This area also does not have a significant naval presence like the Gulf of Aden."
On Tuesday, pirates firing guns and rocket-propelled grenades hijacked an Italian oil tanker in the Indian Ocean and diverted the vessel towards Somalia.
The Irene SL is only the fourth very large crude carrier to have been hijacked by Somali gangs since piracy escalated in 2008, the International Maritime Bureau said.
Joe Angelo, managing director of INTERTANKO, an association whose members own the majority of the world's tanker fleet, said the hijacking of the Irene SL marked "a significant shift in the impact of the piracy crisis in the Indian Ocean."
Angelo said the Irene SL's cargo of Kuwaiti crude oil represented nearly 20 percent of total U.S. daily crude oil imports and urged governments to step up anti-piracy efforts.
"The piracy situation is now spinning out of control into the entire Indian Ocean," he told Reuters.
"If piracy in the Indian Ocean is left unabated, it will strangle these crucial shipping lanes with the potential to severely disrupt oil flows to the U.S. and to the rest of the world."
Pirate gangs are making tens of millions of dollars in ransoms, and despite successful efforts to quell attacks in the Gulf of Aden, international navies have struggled to contain piracy in the Indian Ocean owing to the vast distances involved.
"The situation is only going to worsen. With rising ransoms pirates are able to hire more men, bribe more officials and wait longer periods to negotiate," said AKE's Drake.
"Pirates may not set out into the Indian Ocean with specific intent to capture oil tankers but if they see one they will likely attack it with more determination and ferocity than other vessel types, simply because of the potential ransom sum they could secure through holding such a vessel."
Pirates last year received a record $9.5 million ransom for the release of the Samho Dream South Korean oil tanker. A study showed maritime piracy costs the global economy between $7 and $12 billion a year.
The European Union Naval Force said a South Korean fishing vessel, the Golden Wave, and its 43 crew had been released from pirate control. The ship was hijacked off the Kenyan coast last October. EU NAVFOR did not say if a ransom had been paid.