"Unidentified persons" raided a Singapore-registered tanker some 550 km off the coast of Ivory Coast, officials said on Tuesday.
Twenty crew members of various nationalities were on board the Success 9 when individuals boarded it on Monday in the Gulf of Guinea, according to the Maritime and Port Authority of Singapore (MPA).
The MPA said on Tuesday evening it was working closely with the shipowner, as well as the Monrovia Area Maritime Relief Coordination Center and Singapore's Changi Command and Control Center "to review the situation . and provide the necessary assistance".
No other details were released.
The Gulf of Guinea has long been plagued by acts of piracy, as a key delivery route from Senegal to Angola over 5,700 km. Nigerian criminal gangs are behind most of the attacks.
Since 2021, however, pirates have been carrying out attacks further afield, in international waters, according to carriers.
Their violence and sophisticated techniques have encouraged companies in the sector to call for a larger foreign naval presence as the mission launched a decade ago to stem attacks by Somali pirates.
"It appears that serious attacks are increasing in the Gulf of Guinea. We hope that more international warships equipped with helicopter assets will be able to patrol the area," Noel Choong of the Information Center on Security told AFP on Wednesday. piracy, emanating from the International Maritime Bureau.
In March, pirates seized a Danish tanker off the Congo, kidnapping six crew members before abandoning the ship.