KADUNA, Nigeria - Three suicide bomb attacks on churches rocked a northern Nigerian state Sunday, killing at least 21 people and wounding about 100, officials said, prompting protests in a state that has previously been strained by religious tensions.

The first two blasts occurred within minutes of each other and targeted two churches in the city of Zaria, said Kaduna State police chief Mohammed Abubakar Jinjiri. A third blast hit a church in the city of Kaduna about half an hour later, Jinjiri said.

An official working for a relief agency involved in rescue efforts said Sunday evening that 21 people had been killed and that at least 100 people were hurt. He spoke on condition of anonymity as he said he was not authorized to speak to journalists.

Earlier in the day, Nigerian Red Cross official Andronicus Adeyemo said the Zaria attacks killed the bomber and at least one other person and left 51 wounded. The Kaduna attack claimed 10 more lives, he said, and wounded 29 people.

Jinjiri said security at the three churches prevented the suicide bombers from ramming explosive-laden cars into the buildings filled with worshipers.

Churches have been increasingly targeted by violence. The situation has led churches in the predominantly Muslim north to boost security in a nation of more than 160 million people almost equally divided between Muslims and Christians.

Last weekend, a suicide car bomber detonated explosives outside a church in central Nigeria as gunmen attacked another church in the northeast, killing at least six people and wounding dozens.