Lagos: Suspected separatist militants killed four Nigerian police and two civilians during a gun battle while they were on patrol in the country's southeast, police said on Saturday.

The early Friday morning attack took place in the Ngor-Okpala area of Imo State, where the Indigenous People of Biafra (IPOB) separatist group and its armed wing, the Eastern Security Network (ESN), are active.

Attacks blamed on IPOB have killed dozens of police officers in the last two years in Nigeria's southeastern states, where the group agitates for a separate state for the ethnic Igbo people.

"Four police officers attached to Area Command Ngor-Okpala paid the supreme price having engaged unsuspecting IPOB and ESN militia dressed in black and red regalias in a shoot out," Imo state police said in a statement.

"Stray bullet from the miscreants killed two civilians."

IPOB has constantly denied being behind attacks on police, local government offices and electoral agency buildings.

Separatism is sensitive in Nigeria, where a declaration of an independent Biafra Republic by Igbo army officers in the southeast in 1967 triggered a three-year civil war that left more than one million dead.

Africa's most populous nation is almost equally divided between the mostly Muslim north and the predominantly Christian south, with scores of ethnic groups across the country.

Separatist violence is just one security challenge facing President-elect Bola Ahmed Tinubu as the armed forces battle a 14-year-long jihadist insurgency in the northeast and heavily armed bandit militias in northwest and central states.

Tinubu, a former Lagos governor, comes to office next month after winning a February election marked by technical difficulties, delays and opposition claims of massive vote rigging.