Pirates have seized a Greek-owned oil tanker and its 24 crew off the coast of Togo, a maritime group reports, according to BBC News.

The attackers exchanged fire with a Togolese patrol boat but escaped on the tanker, the International Maritime Bureau says.

Piracy is said to be increasing in West Africa, with six ships seized in 2012. Ships are usually held for several days and the cargo transferred to a waiting tanker, before the crew of the original ship is freed.

This is a different method to that used by Somali pirates, who demand a ransom for the ships they capture and often keep both vessels and crew for many months until they are paid.