Gunmen have killed at least 16 people in the Nigerian city of Kano, while a further five people have been shot dead at a church worship service in the north east in Maiduguri.
The Nigerian Red Cross said Sunday's bomb and gun attack on a theatre used for church services at a university in the Nigerian city of Kano left at least 16 people dead and a dozen more injured.
Al Jazeera's Yvonne Ndege, reporting from Abuja, the Nigeria capital, said: "We know people had gathered for a church service when a major blast went off.
"The areas around the campus have been cordonned off by security forces.
"I must emphasise that it is still unclear and no one has claimed responsibility for the attack."
Lieutenant Iweha Ikedichi, an army spokesman in Kano, said: "A lecture theatre used for worship in the area was attacked." Dozens of students usually attend church services at the theatre at Bayero University on Sundays.
Explosion and gunfire was heard coming from the theatre on Sunday morning. Soldiers cordoned off the area and were not allowing residents to enter.
In Maiduguri, the spiritual home of the armed group, Boko Haram, gunmen shot dead at least five people, including a pastor, who were attending a church worship service in northeast Nigeria, witnesses said.
Police spokesman Samuel Tizhe confirmed the attack, in which witnesses, who declined being named for fear of reprisals, said that the gunmen attacked a Church of Christ.
No group has claimed responsibility for the attacks but our correspondent said that most people suspect Boko Haram of carrying out the attacks.
"It does point to the sect Boko Haram, who have been behind dozens ... of bombings in Nigeria, particularly in the north," she said.
In January, Boko Haram, an armed group seeking the imposition of Islamic law, claimed responsibility for attacks in Kano, the largest city in Nigeria's mainly Muslim north, when co-ordinated bombings and shootings left at least 185 dead.
On Thursday, bomb attacks at the newspaper offices of This Day in the capital Abuja and the northern city of Kaduna left at least nine people dead.
The group has previously targeted churches, including on Christmas day when at least 44 people were killed in a bombing at a church outside the capital.
A bombing on Easter Sunday in Kaduna, near a church, that killed at least 41 people was a stark reminder of the Christmas attacks, but Boko Haram is not known to have claimed it.