File photo by Xinhua/Zhang Baoping

ABUJA, Jan. 2 (Xinhua) -- At least 21 churchgoers returning from a midnight service were feared killed following a mass shooting early Monday in Nigeria's oil-rich state of Rivers, a local official said on Tuesday.

The gunmen allegedly opened fire on a group of churchgoers who had kept vigil in a church to usher in the new year in Ogba-Egbema-Ndoni area of the state early Monday, said Dakuku Peterside, head of the Nigerian Maritime Administration and Safety Agency.

The gunmen were suspected of being members of local cult groups fighting for supremacy, said Peterside, an indigene of the area.

He told reporters that the gunmen shot indiscriminately at the church returnees.

The local police have confirmed the mass shooting, saying an investigation was underway to uncover the identities of the gunmen.

Omondi Nnamdi, a spokesman for the police, said some wounded victims were taken to a hospital for treatment.

Nnamdi said normalcy had since returned to the area following the police's intervention.