A Greek oil tanker with 20 crew members on board had been seized by pirates off Nigeria, Greek authorities said on Monday.

According to a press release issued by the Greek Citizen Protection Ministry, the Liberian-flagged "Aegean Star" of Greek shipping company "Endeavour Marine Agency," came under attack by some 10 armed men on the weekend as it was sailing some 30 nautical miles off Nigeria's coasts.

Greek authorities said the vessel was transporting oil from Ghana to Benin.

Based on earlier information, all crew members, among them the Greek captain and two Greek mechanics, are in good health.

Piracy is an increasing problem in the area, as well as the eastern coasts of Africa over the past three years.

Despite the deployment of international anti-piracy task forces off the coast of Somalia and the Gulf of Aden to safeguard the sea routes of merchant ships, pirates hold dozens of ships and hundreds of hostages.

In many cases shipping companies have opted to pay millions of U.S. dollars in ransoms to secure the release of the crews and the vessels.