Separatist tensions have been on the rise in Nigeria this year, especially in the southeast where calls for a breakaway state evoke sensitive memories of the country's civil war 50 years ago.

The two police were ambushed and killed by gunmen at Isu in the Onicha region of Ebonyi State on Thursday, local police spokeswoman Loveth Odah said, without giving further details.

Dozens of officers have been killed in the southeast this year in attacks authorities blame on the Indigenous People of Biafra (IPOB), a separatist movement agitating for a breakaway state for the ethnic Igbo people.

IPOB denies its Eastern Security Network paramilitary wing is behind the attacks.

On Thursday, the army also said it had detained Chiwetalu Agu, a veteran star in Nigeria's Nollywood film industry, accusing him of fomenting support for the outlawed IPOB in southeastern Anambra State.

"Dressed in a very well known attire of the proscribed group, Chiwetalu Agu was picked up for questioning while inciting members of the public to join," the army said in a statement on its Facebook page, referring to the Biafra half-sun symbol.

A video widely shared on social media appears to show Agu wearing a green, red and black tunic with a rising sun symbol, telling soldiers he was not inciting violence but only distributing bread.

On Friday, Agu was shown in another video posted on social media apparently freed from military detention.

An army spokesman did not immediately respond to a call seeking details on his release.

The incident comes as security forces are tightening controls in Anambra before local elections for the state governor early next month.

"Individuals or groups who fan the embers of violence are warned to desist or face the consequences of their actions," the army said after Agu's arrest.

IPOB leader Nnamdi Kanu is in detention in Abuja facing terrorism and treason charges after he was brought back to Nigeria from overseas in June.

His lawyers say he was illegally extradited from Kenya, though Nigerian officials have given no details.

An activist for more autonomy for the Yoruba people in the southwest was also arrested in July in neighbouring Benin after going on the run from Nigeria.

A declaration of independence by Igbo army officers for a Biafra region in the southeast in 1967 sparked a three-year civil war that left more than one million people dead from fighting or starvation.

pma/ri

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