At least 44 worshippers have been slaughtered at a mosque in northeastern Nigeria, officials in Borno state said Monday.

The killings took place during dawn prayers on Sunday, but the news emerged from the remote area the next day, the BBC reported. It gave the death toll at 44. But the Daily Post of Nigeria said at least 51 people were killed at the mosque and another 12 in a village elsewhere in the state. It said 26 wounded people were being treated in a hospital.

The attack occurred in the town of Konduga, 35km (22 miles) from the state capital, Maiduguri.

The gunmen are believed to be from the Islamic extremist group Boko Haram, which has killed thousands in northern Nigeria since 2009. Agence France-Presse reported that the attacks were believed to be revenge on citizen vigilante groups that are helping the military battle Boko Haram.

In recent weeks, the military has encouraged the formation of vigilante groups to help authorities locate and arrest members of Boko Haram.

The vigilante groups have been credited with reducing the number of attacks, but some have warned that the situation could spiral out of control and lead to further violence.

A member of a civilian vigilante group told the Associated Press that four of their number were killed when they responded to calls for help.

President Goodluck Jonathan declared a state of emergency in three north-eastern states in May as his troops carry out an offensive against the Islamist militants.

Boko Haram wants to overthrow the Nigerian government and create an Islamic state in the north.

News of the attacks came as a video emerged of the group's leader, Abubakar Shekau, claiming responsibility for recent attacks including some that targeted the police and the military.

The Boko Haram leader has been declared a global terrorist by the United States, which in March put a $7 million bounty on his head.

"I'm challenging Obama," Shekau said in the video, AFP reported. He voiced similar challenges to French President Francois Hollande and Israeli Prime Minister Benjamin Netanyahu.

"They are no match for me," he proclaimed.