Pirates have released an oil tanker and its 20 Eastern European crew members seized last week off Nigeria's coast.

Shipping industry officials say all the crew members of the Marshall Islands-flagged ship were freed unharmed early Friday. It is not clear if a ransom was paid to secure their release.

The company that manages the vessel said it would provide information on how the crew was freed as soon as the ship returned to a port.

Pirates seized the tanker on Saturday about 145 kilometers south of Lagos, Nigeria's commercial capital.

Pirates have been carrying out attacks in West Africa's Gulf of Guinea with greater frequency, often targeting vessels carrying oil from Nigeria's oil-rich southern Delta.

In June, the International Maritime Bureau issued a piracy warning for ships traveling near Benin.