Greek maritime officials say pirates have seized a Greek-run oil tanker off the coast of Nigeria.

Officials said Monday that the Aegean Star and its 20-member crew were hijacked on Saturday.

They say the ship was carrying oil from Ghana to Benin when it was seized 55 kilometers off the Nigerian coast, in the Gulf of Guinea.

The ship’s crew is said to include three Greeks and sailors of other nationalities. There has been no word on their condition.

The Liberian-flagged ship is operated by Greece-based Endeavour Marine Company. The company had no comment as of Monday.

While not as well-known as their counterparts in Somalia, pirates have become increasingly active in the Gulf of Guinea.

Last month, the International Maritime Bureau issued a piracy warning for ships traveling near Benin.

It said pirates have seized several vessels in the area this year, including a Greek-owned tanker on June 16.

The IMB’s top piracy official, Noel Choong, said the attacks off West Africa are worrying because they seem to involve a greater degree of violence against crew members.