Four sailors, including 2 Filipinos, were kidnapped and a guard was shot and wounded when suspected pirates stormed an oil tanker off the coast of Togo on Monday, the country's navy said.
In a statement the navy said "armed individuals" launched the overnight attack when the boat was some 10 nautical miles off the coast of the capital Lome.
Armed guards on board the ship tried to repel the raid, it said.
"A member of the armed guard was shot and wounded and the criminals managed to escape, taking four hostages among the crew members: two Filipinos, a Greek and a Georgian," they said.
The navy did not reveal the extent of the armed security on board the ship.
An investigation has been opened into the kidnappings, with the Togolese government vowing to "make every effort to find these criminals".
The latest attack in the Gulf of Guinea -- stretching between Liberia and Angola -- comes just days after nine Filipino sailors in a ship waiting in the harbor off the Beninese port of Cotonou, were abducted by suspected pirates on Saturday.
The West African waters have become one of the most dangerous maritime regions in the world.
Attacks on ships and the abductions of crew for ransom have become more frequent, especially along the Nigerian coast.
The pirates sometimes divert ships for several days, long enough to plunder the cargo and demand huge ransoms before freeing the crew.
Piracy has disrupted the operations of Nigeria and Angola, the two main oil producers in sub-Saharan Africa, and severely disrupted international maritime transport essential to the continent, costing billions of dollars.