The International Maritime Bureau said on Monday that a French-owned oil tanker that went missing off the Ivory Coast over the weekend is believed to have been hijacked by pirates.
The owners of the Luxembourg-flagged tanker with 17 crew on board lost communication with the ship on Sunday, Noel Choong, head of the bureau's Kuala Lumpur-based piracy reporting centre told sources.
"The missing tanker is suspected (to have been) hijacked by pirates," he said, adding the maritime watchdog had issued a warning to ships.
"It appears pirates are moving towards the Ivory Coast because Nigeria and Benin have increased patrols in the Gulf of Guinea," said Choong, who declined to elaborate on the incident.
He expressed concern over a recent spate of incidents in the area, revealing two other attacks on merchant ships in recent days, one of which left a seafarer with gunshot wounds.
Early Monday, a United Arab Emirates-owned chemical tanker was fired on off the Nigerian port city of Lagos but pirates aborted the attack due to the presence of armed security. The injured crew member was taken to hospital.
On January 31, more than 25 heavily armed pirates with machine guns and rocket-propelled grenade launchers in five skiffs attacked a ship off Nigeria.
But despite gunshot damage to the hull of the Nigerian-owned vessel and shattered glass on the bridge, the ship managed to increase speed and escape.
Choong said a suspected pirate mother ship was also sighted during that attack.
With five incidents recorded so far this year, Choong said attacks in the area "are happening more frequently than before."
The Gulf, which includes waters off Nigeria, Africa's largest oil producer, has emerged as a new piracy hotspot, with 62 attacks recorded in 2012 including hijackings, kidnappings and killings.
Pirates usually target fuel cargo, loading it onto other ships to sell on the lucrative black market, rather than seek ransom to release ships.