At least 12 people were killed Sunday when a gunman entered a church and opened fire in southern Nigeria, police said.

20 other people were injured in the attack in Ozubulu in Anambra state, the state's police commissioner, Garba Umar, told journalists.

A witness who asked not to be named said several gunmen stormed Saint Philip Catholic Church during mass, shot a man they appeared to be looking for and opened fire on the rest of the worshippers.

Umar, however, said there was only one attacker.

"From our findings the act was carried out by a native of the area," he said.

"The man, after shooting at his targeted victim, still went on a shooting spree, killing and wounding other worshippers," the police commissioner said.

Umar said the shootings followed a quarrel between two people from the village, who were living in South Africa. Residents said they were drug lords who had returned home to allow community leaders to broker an agreement between them.

"I heard that the matter was being handled by some leaders of the community and it was yet to be resolved. What happened this morning ... came to all of us as a big shock," retired teacher Onyekwere Okafor told dpa.

The first person to be killed in the church was the father of one of the alleged drug lords, said another resident, Abel Amansi.

Umar said that no arrests had been made so far.