Islamist militants have killed about 30 government soldiers in a series of clashes in northeast Nigeria since Wednesday, military and civilian militia sources said on Sunday.
Security across Nigeria has been deteriorating in recent months, including in the northeast.
Four attacks claimed the lives of at least 27 soldiers and 10 members of the Civilian Joint Task Force (CJTF), including a unit head, the sources told Reuters.
CJTF are local groups of armed men who protect the areas where they live, mainly against Islamist insurgents. They are not members of the military but often fight alongside soldiers.
Neither Army nor defence headquarters spokesmen took calls or responded to messages requesting comment.
Islamic State West Africa Province (ISWAP), the regional affiliate of Islamic State, claimed credit for a Friday attack in Monguno, via its Amaq news agency on Saturday.
ISWAP said it had killed 33 soldiers and captured one. Two military sources and one CJTF fighter said the attack, which took place between Monguno and Kukawa, had killed between 11 and 15 soldiers, with several still missing. Four CJTF fighters were also killed.
Soldiers also killed eight militants in fighting in Gamdu on Wednesday, the sources said.
A more than decade-long Islamist insurgency in northeast Nigeria waged by Boko Haram and later ISWAP has displaced about 2 million people and killed more than 30,000.
In Kaduna state in northwest Nigeria, armed men attempted to kidnap more students, a state government official said, as 39 others from an earlier attack remain missing. Four school kidnappings since December have provoked nationwide outrage.