Militants attacked two churches in north and central Nigeria on Sunday, spraying the congregation of one with bullets, killing several people, and blowing up a car in a suicide bombing at the other.

The Islamist group Boko Haram, which says it is fighting to reinstate an Islamic caliphate in mostly Muslim northern Nigeria, has stepped up deadly bombings and shootings against Christian places of worship this year.

This follows its high profile strikes against churches on Christmas Day in December 2011 and on Christmas Eve in 2010.

Experts say the increasing anti-Christian attacks by the sect, which also often targets security forces and establishment figures, appear partly aimed at trying to ignite sectarian conflict in Africa's most populous country of 160 million people, split roughly evenly between Christians and Muslims.

Although Nigeria's Christian and Muslim communities mostly live in peace, periodic flareups of sectarian violence have killed hundreds since independence from Britain in 1960. Many of the recent attacks have targeted the volatile central city of Jos, long a Christian-Muslim flashpoint.

Here is a list of the increasing attacks against Christian targets, especially churches:

Dec. 2010 - At least 80 people are killed in Dec. 24 Christmas Eve bombings, including attacks on churches, around Jos. Boko Haram claims responsibility for the attacks which trigger deadly clashes between Muslim and Christian youths.

Nov. 2011 - At least 65 people are killed in the northeast city of Damaturu when Islamist insurgents bomb churches, mosques and police stations. Boko Haram claims responsibility.

Dec. 2011 - Boko Haram claims responsibility for bomb attacks across Nigeria on Christmas Day, three of which targeted churches, including one near Abuja that killed at least 37 people and wounded 57. President Goodluck Jonathan declares state of emergency in mostly northern parts of the country.

Jan. 5 - Gunmen open fire on church service in Nasarawa in northern Gombe state, killing six people and wounding 10. The attack follows a warning from Boko Haram published in local newspapers that Christians have three days to leave majority Muslim northern Nigeria or they will be killed.

Jan. 6 - Gunmen open fire on Christian mourners at a hall in Mubi in Adamawa state, killing 18. The Christians had gathered to mourn the deaths of three people shot the previous day.

Feb. 19 - Bomb explodes near church in the town of Suleja on the edge of the Nigerian capital Abuja, wounding five people.

Feb. 26 - A suicide bomber drives a car packed with explosives into a church in Jos, killing two people and wounding 38 in an attack claimed by Boko Haram. Christian youths beat two Muslims to death in revenge.

March 11 - Another suspected suicide bomber attacks a Catholic Church in Jos, killing at least three people. Reprisal attacks against Muslims by Christian youths kill at least 10 people.

April 8 - A car bomb explodes on Easter Sunday near a church in the northern town of Kaduna, killing at least 36 people and badly wounding 13.

April 29 - Gunmen open fire and throw homemade bombs at a lecture theatre in the University of Kano being used for Christian worship. A church is also attacked in northeast Maiduguri. A total of 19 people are killed in the attacks, blamed by police on Boko Haram.

June 3 - Suicide bomber drives car full of explosives into church during Sunday service in Yelwa, on the outskirts of the northern city of Bauchi, killing at least 12 people. Boko Haram claims responsibility.

June 10 - Three gunmen spray bullets at the congregation of a church in Biu Town, in northeastern Borno state. In Jos, suicide bomber drives car to the entrance of the Christ Chosen Church and blows it up, witnesses say. Youths attack bystanders in retaliation, killing two, police say.