Islamic State West Africa Province claim twin attacks, which burned down a UN humanitarian hub and police station.
The attacks on Saturday, in the Monguno and Nganzai areas, came just days after armed fighters killed at least 81 people in a raid on a village in a third area, Gubio.
Two humanitarian workers and three residents told Reuters that fighters armed with heavy weaponry including rocket launchers arrived in Monguno, a hub for international non-governmental organisations, at roughly 11am (10:00 GMT).
The fighters then overran government forces, taking some casualties but killing at least 20 soldiers and roaming the area for three hours.
The sources said hundreds of civilians were injured in the crossfire, overwhelming the local hospital and forcing some of the injured to lie outside the facility awaiting help.
The fighters also burned down the United Nations’ humanitarian hub in the area and set fire to the local police station. Fighters distributed letters to residents, in the local Hausa language, warning them not to work with the military or international aid groups.
Fighters also entered Nganzai at about the same time on Saturday, according to two residents and one Civilian Joint Task Force (CJTF) fighter. They arrived on motorcycles and in pick-up trucks and killed more than 40 residents, the sources said.
A military spokesman did not answer calls for comment on the attacks. UN officials could not immediately be reached for comment.
Boko Haram and its offshoot, Islamic State West Africa Province (ISWAP), have killed thousands and displaced millions in northeastern Nigeria.
ISWAP claimed the two Saturday attacks as well as the Gubio attack.