Three deadly blasts have hit churches in the northern Nigerian state of Kaduna in the latest attacks targeting Christian worshippers in the region, emergency services and residents say.

Two of the blasts happened in the city of Zaria on Sunday and another struck the city of Kaduna, leaving a total of at least 21 worshippers dead and dozens wounded. Residents said they feared many people had been killed.

Police and the military cordoned off the areas. Kaduna state authorities immediately imposed a 24-hour curfew.

Al Jazeera's Yvonne Ndege, reporting from Abuja, said the bombings had led to incresing sectarian tension in the streets.

"What we're told is that there have been a number of retaliatory attacks. Young Christians have taken to the streets and have been attaking people. There could be a signifacant number of deaths from all this," she said.

Residents said mobs barricaded roads in the towns of Trijania, Gonin Gora and Sabon Tasha, attacking motorists who looked Muslim.

Nigeria is divided between a predominantly Muslim north and a predominantly Christian south.

Andronicus Adeyemo, of the Nigerian Red Cross, said the Zaria attacks occurred about 10 minutes apart in different parts of the city.

The state-run National Emergency Management Agency (NEMA) said the blasts happened in the Wusasa and Sabon-Gari districts of Zaria, which has previously been targeted by Boko Haram, an armed group which has been responsible for a series of attacks on churches.

"It's obvious to most people here that the violence points to the work of Boko Haram. The group has taken credit for a wave of violence, including attacks on churhes on the last two Sundays," our correspondent said.

"What people are asking is, 'What do they want?'"

Source: Al Jazeera and agencies