The Nigerian army said on Friday that separatists in the southeast killed five soldiers in an attack on a checkpoint.
The main separatist group denied involvement in the attack on Thursday, as the region commemorated the more than one million people who died in the Biafra war and famine half a century ago.
The army blamed the armed wing of the outlawed Indigenous People of Biafra (IPOB) movement, saying it “sprang a surprise attack on the checkpoint” near the city of Aba in Abia State.
“The attack sadly took the lives of five personnel,” spokesman Major General Edward Buba said in a statement.
The army vowed to retaliate, saying it would be “fierce in its response” against the group.
IPOB which wants a separate state for the ethnic Igbo people in southeast Nigeria –routinely denies it is behind attacks, which have also been carried out by criminal gangs and political rivals using the group’s name.
It had issued a strict order for people to remain indoors on Biafra Day and said it had not struck at a “time of mourning”.