Rioting in two southern Nigerian cities Thursday over a police killing forced Muslims to take shelter at barracks over fears they would be targeted in reprisal attacks.

The riots broke out in Onitsha and Asaba after a police officer shot dead a bus driver, police said. Both cities are mainly populated by Igbos, who are overwhelmingly Christian.

Hausas, who are mostly Muslim and often originate from Nigeria's north, took shelter at police and army barracks, though there were conflicting reports over whether any were attacked.

Attacks blamed on Islamist group Boko Haram, some of which have targeted Christians, have sparked fears of reprisals and have led Christian leaders to warn they would defend themselves if the violence continued.

"A police corporal accidentally shot a bus driver dead in Onitsha and this angered the people, who went on the rampage in protests, and the situation caused fear among Hausa residents," Anambra state police spokesman Emeka Chukwuemeka told AFP.

He confirmed that Hausa residents had fled to police and army barracks, but added that none had been attacked following the protests. Chukwuemeka could not say how many fled, but added that the policeman had been arrested.

Delta state police spokesman Charles Muka said the situation also caused panic in nearby Asaba.

"The Onitsha problem spilled over to Asaba as residents ran helter-skelter," he said. "We have drafted our men to secure the town. I am in Abraka, where we have a huge Hausa population, and everywhere is calm."

A spokesman for the main Igbo organisation MASSOB said some Hausas were attacked and various shops were looted. He estimated that between 200 to 300 Hausas sought shelter at barracks.

He said rumours had spread that the policeman behind the shooting was a Hausa and the victim was believed to be an Igbo.

"I cannot say if anyone was killed, but I saw some Hausa people being manhandled by hoodlums who took advantage of the situation to vent their anger on northerners because of what Boko Haram did to Igbos in the north," Madu Uchenna told AFP.

"We have told our people not to seek revenge by killing northerners despite the killings of our kinsmen in the north."

Nigeria, Africa's most populous country and largest oil producer, is roughly divided between a mainly Muslim north and predominantly Christian south.

Boko Haram, which has claimed to be fighting for an Islamic state in Nigeria's north, has been blamed for scores of attacks that have shaken the country.