Pirates attacked two fishing vessels on Sunday near the Gabonese capital Libreville, abducting six crewmen, a source close to the Gabonese government told AFP.
"The pirates kidnapped three Indonesians, two Senegalese and a South Korean," the source said without elaborating.
The Gabonese defence ministry did not respond to requests from AFP for comment.
It was the second pirate attack since the start of the year off the Gabonese coast.
In December pirates attacked several boats off Libreville, abducting four Chinese seamen and killing a Gabonese skipper.
The Gulf of Guinea, which stretches some 5,700 kilometres (3,500 miles) from Senegal to Angola, has become the new world epicentre of pirate attacks, lootings and kidnappings for ransom.
On April 19, pirates attacked a container ship off the coast of the Benin capital Cotonou, leaving eight sailors missing.