Nigerian rebel group Boko Haram has claimed responsibility for the three suicide attacks on churches in Kaduna state on Sunday, saying that the bombings were retribution for past Christian atrocities against Muslims.

At least 21 people were killed in the attacks and 100 injured.

Boko Haram is believed to be behind the series of church bombings over three consecutive Sundays this month. The rebel group, which wants to turn Nigeria into an Islamic state, has often targeted churches during its two-year reign of terror, including the Christmas Day bombings in Abuja in December and the Easter bombings, also in Kaduna.

These attacks have sparked religious tension in Nigeria, and on Sunday a mob of Christians retaliated by killing more than 20 people they believed to be Muslims. In the past, church bombings have been met with attacks on Islamic schools.