"We cannot contact the vessel," an official with Enesel, the Greek company that manages the Irene SL, told AFP by telephone.

The tanker was carrying "about 270,000 metric tons" of Kuwaiti crude, which translates to over 1.9 million barrels of oil, he said, asking not to be named.

The Bahrain-based Combined Maritime Forces said the Greek-flagged ship was hijacked at 0926 GMT about 220 nautical miles (370 kilometres) east of the Omani coast, in the Arabian Sea.

"We can confirm that the Irene SL has been pirated off the coast of Oman," a spokeswoman for the international naval force told AFP by telephone.

"It is an oil tanker," she said, adding that it had a crew of 25 and was "bound for the United States."

"We have no reports of casualties," the spokeswoman said.

While the identity of the hijackers is unknown, Somali pirates are the likely culprits.

"We've got no specific information about who has taken it, but I think it would be reasonable to suspect it was an act of Somali piracy," the spokeswoman said.

The European Union's Atalanta mission to the seas off Somalia and the Gulf of Aden (EUNAVFOR) said the 319,247-tonne ship is both Greek flagged and owned, and said it was travelling from Fujairah in the United Arab Emirates to Suez when it was hijacked.

"She has a crew of 25 (seven Greek, one Georgian and 17 Filipinos). There is presently no communication with the vessel and no information regarding the condition of the crew," EUNAVFOR said.

Enesel's website says the tanker is 333 metres (1,092 feet) long with a 60-metre (196-foot) beam.

Irene SL is the second oil tanker hijacked in as many days.

EUNAVFOR said an Italian oil tanker was taken early on Tuesday 600 miles east of the island of Socotra by a single skiff with five pirates who opened fire on the oiler.

That ship had a crew of 22 -- five Italians and 17 Indians, EU forces said.

Piracy has made shipping increasingly perilous off the Horn of Africa and led to the deployment of an international force to protect the key maritime corridor.

On Wednesday, South Korea's foreign ministry announced that Somali pirates have released a South Korean fishing vessel, the 241-ton trawler Keummi 305, which was hijacked four months ago with 43 sailors aboard.

On Sunday, the Indian navy captured 28 suspected Somali pirates on board a Thai fishing vessel that had been hijacked up to six months ago and was thought to have been used as a floating base to mount attacks on shipping.

In January, pirates released a Greek-owned oil tanker with a crew of 18 Filipinos that they had seized in the southern Red Sea.

The UN's maritime agency, the International Maritime Organisation, said last week that 67 ships had been hijacked off the coast of Somalia in the past 12 months alone, while a total of 714 seafarers are still being held for ransom on board 30 ships along the eastern African country's extensive coastline.

London's Chatham House international affairs think-tank estimates that piracy costs the global economy between $7 billion and $12 billion (five billion and 8.8 billion euros) every year.

Leading global shipping groups have called for a "more robust" international response to Somali piracy, warning that escalating violence towards seamen could prompt the industry to seek alternative routes.

"The current situation is unacceptable to the industry," four shipping associations said in an open letter dated February 4, released Monday by the union of Greek shipowners.

"Unless necessary action is taken by the international community, the shipping industry will be looking at all possible options, including alternative routes, which could have a very dramatic effect on the world economy and global trade, including the delivery of oil," the groups warned.