A rescue mission by Nigerian forces off Benin has found no trace of eight missing seafarers on a Maersk-chartered boxship, leaving open the question of how they were abducted.

The 4,957-teu Tommi Ritscher (built 2014) was boarded by gunmen on Sunday morning at anchor off Cotonou.

Eleven seafarers took refuge in the citadel, but another eight were unaccounted for.

They were said to be being held as hostages on board after a Benin navy patrol scared off the speedboat the pirates arrived on, leaving some of the perpetrators stuck on the ship.

But by the time of the Nigerian special forces' assistance on Monday evening, together with Benin naval personnel, there was no sign of them or the attackers.

The Nigerian operation was unopposed, security consultancy Dryad Global said.

The ship is owned by Woollerton Shipping and managed by Transeste Schiffahrt of Germany.

The companies confirmed on Wednesday that the eight crew members who did not reach the safety of the citadel have been abducted.

The kidnapped seafarers include Bulgarian, Ukrainian, Russian and Filipino nationals.

The 11 seafarers confirmed safe remain onboard under Benin navy guard.

"Arrangements are being made for the vessel to be taken to port to ensure its security as the crew undergoes further debriefing and welfare checks," the companies said.

Unusual incident

Dryad said: "This is a highly irregular incident for which there are only two rational courses of action to explain the absence of crew.

"The first being that earlier reporting regarding the perpetrators being abandoned on the vessel were false and that the eight crew members were kidnapped in the early stages of the incident.

"The second narrative is that the perpetrators and eight crew members were at some point able to disembark the vessel and escape."

The vessel had remained under observation by Benin naval ships, however.

"It would be considered a serious operational failing if the perpetrators were able to escape given the significant military presence in the vicinity of the vessel and thus it is assessed as a realistic possibility that communications from within the citadel were incorrect and that the perpetrators were able to escape at an early stage of the attack with eight crew members," Dryad added.

Nigerian gang likely responsible

The company believes it is likely the pirates are Nigerian in origin.

"It is assessed that the perpetrators are likely to have been aware that Benin had taken the decision to cease placing guards on board vessels at anchor as a result of Covid-19 and were aware of the limited established security presence within the area," it said.

"Whilst the full details of the incident remain unknown, incidents within the anchorage area have predominantly been limited to suspicious approaches and boardings for the purposes of petty theft."

According to Clarksons, Tommi Ritscher was chartered to Maersk Line in early April 2019 for between 10 to 17 months at $15,250 per day.