Pirates boarded a Swedish tanker off the coast of Benin at the weekend but were chased away by the African nation's navy, leaving the vessel and crew unharmed, the ship's owner said Monday.

"Protection forces from the Benin navy came out to the ship after the crew reported the attack and scared away the pirates," Jonas Engström of Wisby Ship management told AFP.

Pirates boarded the 183-meter (600-feet) Gotland Sofia overnight to Sunday, causing the crew of 23 to retreat to a fortified room, or so-called citadel, on the ship.

Engstroem said he did not know how long the pirates were onboard, but the crew stayed in the safe room "for about four or five hours" until the incident was considered over and it was certain no pirates remained onboard.

"Everyone is physically unharmed and no one had direct contact with the pirates," he said.

The ship was carrying fuel and travelling from Norway to a ship-to-ship operation to be held off Benin's coast.

"The vessel has now been escorted closer to the port for a thorough search by the military and investigation of the incident," the company said in a statement, adding the vessel suffered only minor damage.

Engstroem said pirate attacks in the area had become increasingly frequent in the area.

Last week, pirates seized an Italian tanker with a crew of 23 off Benin in the Gulf of Guinea but released the crew unharmed days later.

Earlier in July, the Liberian-flagged Greek oil tanker Aegean Star, which had been hijacked by gunmen off the coast of Nigeria, was released along with its 20-strong crew.