Pirates who seized a Danish oil tanker off Congo last week have kidnapped six crew and abandoned ship, the French navy said Friday.

The Monjasa Reformer had 16 sailors when it was boarded by pirates on March 25.

The remaining crew were rescued by a French naval ship. They said "six crew members had been kidnapped by the pirates", the French navy said in a statement.

The Liberian-flagged tanker was found on Thursday off the coast of Sao Tome and Principe in the Gulf of Guinea.

It had been spotted by a drone from the French patrol boat Premier Maitre L'Her, deployed as part France's anti-piracy mission Corymbe, the French defence ministry said.

The pirates' boat, which was alongside the tanker during a first flyover, was not there later.

The French navy said three of the remaining crew members had received "minor injuries" and had received treatment.

In an earlier statement, shipowner Monjasa said "the pirates had abandoned the vessel and brought a part of the crew members with them," without specifying their number or nationalities.

"Our thoughts are with the crew members still missing and their families during this stressful period," Monjasa said, adding that it was "working closely with the local authorities" to obtain the sailors safe return.

- Communications lost -

Danish news agency Ritzau said that none of the crew were Danish.

The owner said there was no reported damage to the vessel or cargo.

The 135-metre-long Monjasa Reformer "experienced an emergency situation" on March 25 around 160 miles (260 kilometres) west of Port Pointe-Noire in the Republic of Congo when it was "sitting idle", Monjasa said.

The crew had notified the company that pirates had boarded and that the entire crew was safe in the tanker's secure room or "citadel", "in accordance with the onboard anti-piracy emergency protocol".

It said communications were then lost.

The Maritime Domain Awareness for Trade in the Gulf of Guinea, a cooperation centre between the British and French navies, and the French navy said five armed people boarded the vessel after approaching from a skiff.

Earlier this week, an official at the port of Pointe-Noire had put the number of attackers at three and said it was in international waters when the pirates boarded.

Monjasa gave no details about the number of pirates or their nationalities.

- Lull in activity -

Pirates have long been a risk in the Gulf of Guinea -- a major shipping route stretching 5,700 kilometres (3,500 miles) from Senegal to Angola, with Nigerian gangs carrying out most attacks.

But since 2021, shippers say pirates have been raiding farther out in international waters.

Their violence and sophisticated tactics prompted pleas from shippers for a more robust foreign naval presence like the mission to curb attacks from Somali pirates a decade ago.

Many of the attacks in recent years have been carried out by Nigerian criminal gangs using speed boats from hideouts in the Delta region.

Some gangs have captured larger fishing vessels which they use as a "mothership" base to raid further out to sea.

But the region, which sees a lot of traffic from oil tankers, has also seen a lull in activity recently.

According to a report by The Maritime Information Cooperation and Awareness Center (MICA), three ships were attacked in the area in 2022 compared to 26 in 2019.

Two other attacks have been recorded in the region in 2023 so far.