The International Maritime Bureau said the pirates took control of the ship's bridge upon entering and engaged naval men in a gun duel before escaping with the passengers.
The ship owned by a Greek company which anchored 17 nautical miles off Lome, the capital Togo, sounded a signal upon noticing the intrusion which attracted the Togolese navy, an official of IMB, Noel Choong, said.
He said a similar attack took place a week earlier during which the pirates stole all the oil on the vessel and left the sailors to go.
Most attacks on the Gulf of Guinea are carried out by Nigerian militants who became increasingly sophisticated after forming several groups that operated within Nigeria's coast for decades.
The groups known as Niger Delta militants, extended their operations beyond the country
