The Maran Cantaurus was travelling slowly when it was attacked
Somali pirates have captured a tanker carrying oil to the US, officials say.
The Greek-owned Maran Centaurus was about 1,300km (800 miles) off Somalia when it was hijacked on Sunday, said the EU Naval task force (Navfor).
The ship was full of oil and is believed to be one of the largest yet seized by Somali pirates. There are 28 crew members on board.
Pirate attacks have been common off the Somali coast and international navies have been deployed to counter them.
A spokesman for the Greek coastguard told Reuters news agency that about nine armed pirates attacked the ship close to the Seychelles.
As it was fully laden, it was moving quite slowly - between 11 and 15 knots (20-27km/h) - when attacked, a Navfor spokesman told the BBC.
Reuters reports the Greek defence ministry as saying that a Greek navy frigate which had been involved with the Navfor operation was now shadowing the vessel.
Navfor said the ship, which has a dead weight of some 300,000 tonnes, had been sailing to New Orleans in the US from Jeddah in Saudi Arabia but was now heading towards Somalia.
Its crew is made up of 16 Filipinos, nine Greeks, two Ukrainians and one Romanian.
Maran Tankers Management, which operates the vessel, told Reuters the crew were "well".
Somalia analyst at the International Crisis Group think-tank Rashid Abdi says the fact that pirates are now operating so far out to sea shows that the intervention of the world's navies has made little difference to the problem of piracy.
"This incident clearly shows the pirates are becoming more bolder," he says.
"So I don't think the solution is in building the naval deployment there, or increasing the naval deployment. The problem is actually in dealing with the governance crisis which feeds the problem of piracy."
War-torn Somalia has had no functioning government since 1991, allowing pirates to operate along the lawless coast almost with impunity.
In recent months, the pirates have started operating further from the Somali coast.
They are believed to use "mother-ships" to reach the high seas, before using small skiffs to carry out their attacks.
Pirates are currently holding 11 vessels and 264 crew members in Somalia, the Navfor spokesman said.
In November 2008, the Sirius Star, carrying two million barrels of oil - a quarter of Saudi Arabia's daily output - became the largest ship ever seized by pirates. It has a deadweight of 318,000 tonnes.
The vessel was released in January after a ransom of $3m (then £1.95m) was paid.
Navfor is one of several international naval forces patrolling the oceans off the country to try to prevent the captures of ships using the vital sea routes. Nato and the US also lead task forces.
Earlier this month, the US began using unmanned drones to scour the Indian Ocean for suspect vessels.