A radical Islamist sect has claimed responsibility for three church attacks that left at least 21 people dead in northern Nigeria.

The group known as Boko Haram made the announcement in an email statement received by The Associated Press on Monday.

This is the third weekend in a row that the group is claiming responsibility for church attacks across Nigeria.

It claimed responsibility for attacks in the first and second weekends of June that killed at least 21 people.

Sunday's attacks hit the city of Zaria and the nearby city of Kaduna in a religious flashpoint state.

Churches have been increasingly targeted by violence in Nigeria, a nation of more than 160 million people almost equally divided between Muslims and Christians.

Boko Haram is held responsible for more than 580 deaths this year alone according to an AP count.