The governor of Imo state, Hope Uzodinma, said a curfew had been imposed in the city of Orlu and nine surrounding districts on Monday after "militants... unleashed a shooting spree... killing an innocent citizen."

"As an interim measure to immediately restore the peace in Orlu zone, a dusk-to-dawn curfew (6pm to 6am) is hereby imposed," he said in a statement.

The region has long been a hotbed for separatist groups, and tensions between the federal security forces and the indigenous Igbo population have been high.

Videos and pictures on social media appeared to show soldiers shooting in the streets of Orlu, but the images could not be immediately verified independently.

Local residents also reported several people were killed and wounded but those reports were not immediately confirmed by officials or police.

The reason behind the clashes was unclear, but one Orlu resident told AFP the army retaliated after a soldier was allegedly killed by militia members.

"The incident started on Sunday when security operatives clashed with some youths and militia members following the reported killing of a soldier," said resident Felix Chiazor.

"Angered by the incident, the soldiers returned to Orlu and met stiff resistance, leading to sporadic gunshots in the area and burning down of houses and shops."

The militia involved is the Eastern Security Network, which is backed by the Indigenous People of Biafra (IPOB) -- a political movement that seeks a separate state for the Igbo people of southeast Nigeria.

Human rights bodies have long accused Nigerian security forces of killing IPOB members, claims that they have denied.

Police spokesman Orlando Ikeokwu confirmed on Tuesday that "clashes" erupted in Orlu but would not provide further details.

"Normalcy is returning (to the area)," the police officer said, adding that officials would comment later on unconfirmed reports of people injured and killed during the clashes.

A traditional chief from Orlu, Ernest Adimiaka, told AFP that four people including a middle-aged woman had died in the clashes.

"We are yet to confirm the identity of those killed but we saw their lifeless bodies on the street. This is aside the scores of people that sustained various forms of injuries," Adimiaka said.

Calls for a separate state of Biafra are a sensitive subject in Nigeria, after a unilateral declaration of independence from British rule in 1967 sparked a brutal 30-month civil war.

More than one million people died, most of them Igbos, from conflict, hunger and disease.

The recently-formed militia is "scattered all over the southeast," said Don Okereke, an independent security analyst who specialises in the region.

"Its members are training like military, it's worrying," said Okereke. "Some radical elements believe that if they are not armed, the military can take over at any time."