The crew of a Marshall Islands-flagged oil tanker seized by pirates last week off the coast of Nigeria has been released.

The Russian agency that hired the sailors said the entire crew was freed early Friday, and that none of the members had been harmed.

The company said it would provide information on how the crew was freed as soon as the vessel returned to 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.