A couple and two others were taken around 02:00 local time last Friday.

A local chief told the BBC they had been living in a rural community providing free medical aid for up to 10 years.

Police spokesman Andrew Aniamaka said no ransom demand had yet been made.

He said militants, calling themselves the Karowei, were the main suspects.

Kidnapping for ransom is common in the oil-rich Niger Delta.

Police believe this incident could be a response to a recent surge in efforts to tackle militancy in the region.