(CNN) -- At least 22 people have been killed in three separate attacks in northeastern Nigeria since Friday, including 15 Christians shot Sunday inside a church, according to officials.
The violence began early Friday when unidentified gunmen raided a village in Musari, in Borno state, killing five people, said Joint Task Force Lt. Col. Sagir Musa. JTF troops responded, and a gunbattle ensued. Three of the attackers were killed, and troops recovered weapons and ammunition. Musa declined to provide further details of the attack.
Also Friday, gunmen with suspected ties to the Islamist militant group Boko Haram killed two people and wounded another in an attack in Maiha, in Adamawa state, according to Godfrey Okeke, Adamawa state commissioner. The attackers freed 35 inmates from the Maiha prison and set government buildings on fire, Okeke said.
In Sunday's attack, gunmen entered a church in Chibok, also in Borno state, and killed at least 15 worshipers, said Mohammed Kana, a regional official for the National Emergency Management Agency.
"Some of the people had their throats slit," Kana said, citing NEMA staff who responded to the scene.
Sunday's violence comes six days after attackers raided two churches during Christmas Eve services, killing 12 people.
In October, a report from Human Rights Watch addressed the violence plaguing northern Nigeria, particularly from Boko Haram. The group's name means "Western education is forbidden." It seeks to impose a strict version of Sharia law in the Muslim-dominated northern part of the country.
"Suspected members of the group have bombed or opened fire on worshipers in at least 18 churches across eight northern and central states since 2010. In Maiduguri, capital of Borno state, the group also forced Christian men to convert to Islam on penalty of death," it said.
It is not immediately known whether the group was behind the latest attacks.