Nine Chinese and eight Ukrainian seamen were abducted on Thursday (Aug 15) when two merchant vessels came under attack in Cameroonian waters, sources said Friday.

"Seventeen Chinese and Ukrainians were kidnapped ... (of whom) nine (are) Chinese who were abducted on one of the ships," an official in the port of Douala told AFP.

A Cameroonian security official, likewise speaking on condition of anonymity, confirmed the account.

The abduction was reported on Thursday by sources in the Cameroonian navy and the country's port service, but their number and nationality were not then known.

The navy source had said the kidnappers "are probably Nigerian pirates", adding that Cameroon's security forces had launched a search for them.

The Gulf of Guinea, whose coastline stretches in a huge arc from Liberia to Gabon, is notorious for piracy as well as oil theft, illegal fishing and human and drugs trafficking.

In Malaysia, Noel Choong, who heads the International Maritime Bureau (IMB), a watchdog agency, said the 17 seamen were seized from two ships that were attacked within hours of each other while they were anchored off Douala.

Choong said one of the ships was a multipurpose German-owned ship that flew the flag of Antigua and Barbuda.

"Eight crew were kidnapped from the ship consisting of a total of 12 Asian and European sailors," he said.

The other vessel was a Liberian-flagged bulk carrier managed in Greece with a Greek owner.

"There were 21 crew on board. All were Asians. Nine crew were taken," Choong told AFP.

"(The) IMB has issued a warning to all ships at Douala. We ask all ships to take additional precaution."

According to the IMB's figures, 62 seafarers were taken hostage or abducted in the area in the first half of 2019.

The Gulf of Guinea accounts for 73 per cent of kidnappings and 92 per cent of hostage-takings at sea worldwide, it says.

The 17 countries in the Gulf of Guinea and adjacent regions have limited surveillance and maritime defence capabilities.

They have been trying for several years to bolster their means of intervention and to put in place closer collaboration. (AFP)