Gunmen dressed as priests have killed three Nigerian soldiers in the oil-producing Niger Delta, a military source and residents have told Reuters.
The attack happened in the Nembe area in Bayelsa state, the source said, noting that the military was searching for the men.
The area has been a flashpoint for violence since tensions arose in the early 1990s between indigenous people and foreign oil companies. Groups including the Ogoni and Ijaw believe they are being exploited and that they are not being rewarded by central government for the use of their resources. An amnesty programme has seen a sharp reduction in violent incidents.
Suspicion for the latest attack is likely to fall on a new group, the Niger Delta Avengers (NDA), which announced its existence in March 2016. The group has attacked oil producing facilities, causing a fall in Nigeria's oil production to its lowest level in 20 years.
The NDA aims to create an independent state in the Delta, where oil extraction has been held responsible for widespread human rights violations, corruption and massive environmental degradation.
While the Nigerian army is able to control the region by force, the NDA is regarded as a major irritant for Nigeria's President Buhari. There are fears that if, as expected, the amnesty programme and its associated social investment come to an end, support for the NDA will grow.