A supertanker carrying about $200m (£125m; 146m euros) worth of crude oil has been hijacked off the coast of Oman, the vessel's Greek operator says.
Athens-based shipping company Enesel said they had lost communication with the Irene SL.
The 333m (1,093ft) vessel was on its way from the Gulf to the Gulf of Mexico when it was attacked.
Although the incident happened hundreds of miles from Somalia, pirate gangs are known to operate there.
A body representing the owners of much of the world's tanker fleet warned that - unless governments do more to combat Somali pirates - global oil-supplies could face severe disruption.
"This morning the vessel was attacked by armed men," said Enesel in a statement quoted by Reuters.
"For the moment there is no communication with the vessel."
Greece's Merchant Marine Ministry told the Associated Press that the ship was carrying 266,000 tons of crude oil.
It is believed to be one of the largest vessels ever seized.
It has a 25-member crew including seven Greeks, 17 Filipinos and one Georgian, according to the ministry.
The EU's naval mission in the region Eunavfor said in a statement on its website the Irene was sailing 400 miles (650km) south-east of Muscat.
Joe Angelo, the head of the International Association of Independent Tanker Owners said that the tanker's cargo represented approximately 20% of one day's US crude oil imports.
"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 US and to the rest of the world," Mr Angelo told Reuters.
The incident comes a day after pirates took control of an Italian oil tanker in the Indian Ocean, some 800 miles from Somalia's coast.
Before the latest incident, Eunavfor said pirates were currently holding 29 vessels along with an estimated 681 hostages.
Somali pirates have made millions of dollars in recent years by capturing cargo vessels in the shipping lanes around the Horn of Africa and holding the ships and crew for ransom.
Somalia has had no functioning central government since 1991, allowing piracy to flourish off its coast.