Militants attacked two churches in 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, injuring 41, witnesses said.
No one was killed by the car bombing in the central city of Jos, but mobs of youths attacked bystanders in retaliation, killing two, police said.
There was no claim of responsibility. Attacks on churches have become a trademark tactic of Islamist group Boko Haram, which says it is fighting to reinstate an ancient Muslim caliphate in northern Nigeria.
Hamidu Wakawa was at the church in Biu Town in northeastern Borno state when it was attacked. "Three gunmen came to the premises of the church and started firing at people outside the church before going into the main building to carry on their killings," he said.
"Many people have been killed and wounded."
To the southwest, in the city of Jos, a man drove a car to the entrance of the Christ Chosen Church and then blew it up, said Emmanuel Davou, 53, who lives nearby.
Emmanuel Ayeni, Police Commissioner for Plateau state, of which Jos is the capital, , told journalists 41 people were being treated for injuries in a local hospital.
"The circumstances of the two killed by mobs is still unclear," he said.
Boko Haram has been blamed for hundreds of killings in bomb or gun attacks over the past two years.
Its leader, Abubakar Shekau, frequently justifies attacks on Christians as revenge for killings of Muslims in Nigeria's volatile "Middle Belt", where the largely Christian south and mostly Muslim north meet.
It was too early to tell if the attacks were coordinated. Security sources say Boko Haram has many different factions that sometimes seem to operate independently.
Moments after the suicide bombing in Jos, Christian youths set up roadblocks and had to be dispersed by police.
"Angry youths have gone wild, even attempting to prevent the security personnel from getting to the scene of the incident. They had to force their way out by shooting in the air to disperse them," said Davou.
Boko Haram has linked up with other Islamist groups in the region including al Qaeda's north African wing and has become the biggest security threat in Nigeria, Africa's top oil producer.
It usually targets security forces, although Christian worshippers are increasingly bearing the brunt.
Last Sunday, a suicide car bomber killed at least 12 people at a church in the remote northern town of Yelwa.