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.