ABUJA, Nigeria (AP) — Gunmen killed at least 15 people on Tuesday during attacks on two villages in northwestern Nigeria, authorities said.

The villages of Falale and Kadobe — neighboring communities in the Jibia area of Katsina State — were attacked in the early afternoon, according to Nasir Mu’azu, the state’s Commissioner for Home Affairs.

Mu’azu said local security forces had previously killed three gunmen during a firefight in the area. In retaliation, gunmen carried out a reprisal assault on Tuesday that left at least 15 people dead.

“Security forces have since restored order and stabilized the situation,” he said in a statement. “We appeal to residents to remain calm and allow security forces to complete their investigation.”

No group has claimed responsibility for the attacks. Armed groups, known as ‘bandits’, regularly carry out raids and kidnappings for ransom in the northwest and north-central part of Nigeria.

Authorities have said the bandit groups include mostly former herders who took up arms against farming communities after clashes between them over increasingly strained resources.

Alongside attacks by bandits, Nigeria is also plagued by an insurgency fought by the Boko Haram extremist group and its splinter faction, the Islamic State West Africa Province. Both groups are mostly active in northeastern Nigeria.

On Monday, suspected suicide bombings killed at least 23 people and wounded 108 others in the northeastern city of Maiduguri.

The security crisis in Africa’s most populous country has worsened recently to include other militants from the neighboring Sahel region, including the Jama’at Nusrat al-Islam wal-Muslimin, which claimed its first attack on Nigerian soil last year.

Several thousand people in Nigeria have been killed, according to data from the United Nations. Analysts say not enough is being done by the government to protect its citizens.

The U.S. sent troops last month to the West African nation to help advise its military on the fight against insecurity.