Members of Boko Haram, an Islamist armed group from Nigeria, have stormed a prison in Yobe state in the country’s northeast, freeing 40 inmates, police have said, in the latest of a string of attacks blamed on the group.

"There was an attack on the Yobe prison this morning, the Boko Haram sect attacked the prison through the Emir palace, 40 inmates escaped," Patrick Egbuniwe, Yobe State police commissioner, said on Sunday.

"They attacked with rifles, the police and the joint task force confronted them and the Boko Haram members that were shot were carried away by the sect."

He said two of the attackers were shot dead and some policemen were injured.

Gun battles between Boko Haram members and security forces in Yobe's capital Damaturu killed 40 people on Tuesday last week. Boko Haram has been blamed for several prison breaks in the past and one of the group's few demands has been that its imprisoned members should be freed.

Uneasy calm

Attacks on churches have also intensified this month, sparking deadly religious violence in northern Kaduna state.

The government has now placed a curfew on Kaduna, which has quelled fighted in some parts but, speaking to Al Jazeera's Yvonne Ndege, many residents said that they felt the curfew would do little to address the dangers posed by Boko Haram. 

The group, which is based in the remote northeast, has rapidly overtaken rebels in the oil-producing southern Niger Delta as the country's biggest security threat. 

It is fighting to carve out an Islamic state in Nigeria – a country nearly evenly split between Muslims and Christians.

Goodluck Jonathan, the Nigerian president, said Boko Haram was seeking to incite a religious crisis by attacking churches in an attempt to destabilise the government.

"Terrorists all over the world have one common agenda: destabilising government," he said during a question-and-answer session on national television.

Months of gun and bomb attacks blamed on the Islamist group have killed hundreds since the movement started its uprising more than two years ago.