COTONOU, BENIN (AFP) - Pirates attacked an oil tanker off the coast of west Africa, killing a Ukrainian seaman, the commander of Benin's naval forces told AFP Tuesday.

"The death that we have is the chief mechanic, who is responsible for the engines. He is Ukrainian. The captain, a Lithuanian, is not hurt," naval Commander Maxime Ahoyo told AFP.

Initial reports suggested the captain of the Monrovia-flagged Cancale Star had been killed and his first officer seriously wounded when pirates boarded the vessel 18 nautical miles off the Benin coast.

However, Ahoyo said the captain, a Lithuanian, was unharmed.

He said the 230-metre long tanker was attacked overnight Monday.

There were 24 seamen of different nationalities on board the vessel, he said. It was unclear whether pirates were still aboard the ship, amid reports that one of the pirates had been overpowered by the crew. Ahoyo said a naval patrol "is currently at sea to investigate the situation."

Piracy in west Africa is on the rise, according to the International Maritime Bureau, with more than 100 cases last year.

Most attacks occur while ships are at anchor or close to the shore, unlike in east Africa, where Somali pirates have netted millions of dollars in ransoms in exchange for the release of ships captured hundreds of miles from the coast.

The IMB's Live Piracy Report has highlighted the risk to shipping in Tema, Ghana, and in the Lagos and Bonny River areas of Nigeria, immediately east of Benin.

It said that pirates have attacked and robbed vessels and kidnapped crews along the coast and rivers, anchorages, ports and surrounding waters.