Jihadist group Islamic State said it killed 13 Nigerian soldiers and five troops from a West African anti-militant force in attacks over recent days.
Islamic State West Africa Province (ISWAP), which split from Nigeria-based Boko Haram in 2016, carried out a series of attacks in the last few months.
In its newspaper Al-Nabaa, IS said its fighters killed the Nigerians in attacks on a military barracks on Friday, a military post on Sunday, and a town on Monday, all in Borno state.
Fighters also detonated explosives on Wednesday on a vehicle in the Lake Chad region, killing five more soldiers, IS said, without specifying the country.
The militants said the vehicle was carrying troops from a multinational task force – comprising troops from Nigeria, Niger, Chad and Cameroon – set up to fight them.
A Nigerian military spokesman declined comment and a spokesman for the task force could not immediately be reached.
Boko Haram has waged a decade-long insurgency in north-east Nigeria which killed around 30,000 people and forced about two million to flee their homes.