Armed men killed at least 11 people in a surprise attack in Nigeria's southeastern Abia state, the military said on Friday, the latest in a string of raids in a region rife with separatist violence.

The assailants killed five soldiers deployed as peacekeepers in the area and six civilians in the crossfire, defence spokesperson Major-General Edward Buba said.

No group claimed responsibility, but the army blamed the outlawed separatist Indigenous People of Biafra (IPOB) movement which wants the surrounding region to secede from Nigeria.

The attack occurred at a military checkpoint at Obikabia junction in Obingwa local government area on Thursday, Buba said in a statement.

"The military will be fierce in its response. We will bring overwhelming military pressure on the group to ensure their total defeat," he added.

The unrest in the southeast has piled pressure on a government and military already struggling to contain attacks and kidnappings in the northwest, a 15-year Islamist insurgency in the northeast and sectarian and herder-farmer clashes in central regions.

IPOB campaigns for the secession of southeastern Nigeria where the majority belong to the Igbo group.

The movement's leader - Nnamdi Kanu, a British citizen arrested in Kenya in 2021 - is now on trial in Nigeria on terrorism charges.

More than a million people died, mostly from starvation, during a three-year civil war in the late 1960s when the region attempted to secede under the name of the Republic of Biafra.

(Writing by Elisha Bala-Gbogbo; Editing by Andrew Heavens)