Officials on Monday confirmed the hijacking of the Greek-owned Maran Centaurus, which was carrying crude oil from Saudi Arabia to the United States.
Somali pirates have seized an oil tanker, with 28 crew members on board, in the waters around East Africa.
Commander John Harbour, a spokesman for the EU naval force patrolling the region, said on Monday that the 300,000-tonne ship had been hijacked the previous day off the coast of Somalia.
The ships 28-member crew is thought to consist of 16 Filipinos, nine Greeks, two Ukrainians and one Romanian.
Officials could not immediately say how many barrels of oil were on board, but its value would be in the millions of dollars.
Although pirates have successfully hijacked dozens of vessels in the last several years, Sunday's attack appears to be only the second ever on an oil tanker.
"This is a jackpot to the pirates," Andrew Mwangura, from the East African Seafarers Assistance organisation in Nairobi, told Al Jazeera.
In November 2008, pirates hijacked the Saudi supertanker Sirius Star, which held two million barrels of oil valued at about $100m.
The tanker was released last January for a reported $3m ransom.
Piracy in the region has increased despite the ramping up of international navies patrolling the Indian Ocean and Gulf of Aden, one of the world's busiest shipping routes.
Somali pirates have seized the Greek-owned ''Maran Centaurus'', an oil tanker with 28 crew members on board, yesterday, according to reports. 

Officials say the tanker, which can transport more than two million barrels of oil, was sailing from Saudi Arabia to the United States when it was hijacked. A tanker full of oil could be worth millions of dollars, and possibly pose a significant security and environmental threat. Authorities, however, couldn't immediately ascertain how much oil the vessel was carrying. 

The ship's crew consists of sixteen Filipinos, nine Greeks, two Ukrainians, and one Romanian, the Al Jazeera news agency reports.

Somali pirates have hijacked dozens of ships over the past two years, demanding and receiving millions of dollars in ransom from the hijacked ships' owners.