Islamic State said via its Amaq news agency it killed or wounded more than 40 soldiers in Borno State in two separate attacks.
The group said militants attacked a military post in Baga and killed at least 15 soldiers before a second attack on an army barracks in Benisheik, where they killed around 25 more.
Local and military sources told Reuters there were clashes between insurgents and soldiers on Monday and Tuesday. Reuters was unable to immediately verify the number of people killed or injured in those clashes.
An army spokesman did not immediately respond to requests for comment.
More than 30,000 people have been killed in north-east Nigeria since 2009 in an Islamist insurgency.
At the weekend at least 65 people were killed in Borno, birthplace of the insurgency, when militants targeted civilians coming from a funeral.