PLATEAU STATE, Nigeria (CNN) -- Gunmen attacked a mosque in Nigeria with automatic weapons over the weekend, killing at least 44 people, a local police official said.

It's not clear who was behind Sunday's massacre, but the militant Islamist group Boko Haram -- which has staged attacks in the same region before -- issued a video around the same time boasting that it was gaining in strength.

Details have been slow to emerge about the attack in Konduga, about 30 kilometers (almost 19 miles) southeast of Maiduguri, the capital of Nigeria's northern Borno state.

The police official in Konduga spoke to CNN on condition of anonymity because he is not authorized to speak to the media.

Another 26 victims were brought into Maiduguri Teaching Hospital for treatment on Sunday, some in critical condition, a source at the hospital who asked not to be identified told CNN.

Some villagers in Borno state have also been attacked in their own homes, accused of collaborating with government forces against Boko Haram, police sources said.

Local media reported that at least 12 people were killed on Saturday night in Ngom village alone.

In the video released by Boko Haram, its leader Abubakar Shekau claimed that the militant group is growing stronger despite the Nigerian military's efforts to curb its activities.

"You soldiers have claimed that you are powerful, and that you have defeated us, you say we are mad people; but how can a mad man organize the attacks in Gamboru, in Malumfatori, slaughter people in Biu, kill in Gwoza and in Bama successfully, where soldiers fled under our heavy fire power?" he said.

"We have killed countless soldiers and we are going to kill more. Our strength and firepower is bigger than that of Nigeria. Nigeria is no longer a big deal to us, as far as we are concerned.

"We will now comfortably confront the United States of America."

Shekau said the group would continue to kill unbelievers and "anyone who stands against the will of Allah by opposing Sharia," or Islamic law.

Boko Haram has often targeted Christians in Nigeria but the group will also attack other Muslims when it feels they are not adhering to strict Sharia law.

There have been suggestions that the group has attacked certain mosques because worshippers at that mosque have assisted government forces in tracking members of the militant group.

'Contemptible act'

The U.S. State Department named Shekau as a "specially designated global terrorist" in June last year. This year, it offered up to $7 million for information leading to his arrest or conviction, under its Rewards for Justice Program.

Britain and Spain each condemned the attack in Borno state and offered their condolences Tuesday to the victims and their families.

"This senseless attack has reportedly killed 44 people and left many others injured. Attacking innocent people in a place of worship is a contemptible and cowardly act," said Mark Simmonds, UK Foreign Office Minister for Africa.

"The UK stands with the government and people of Nigeria as they seek to reduce violence in the north east of the country."

Spain's Foreign Ministry "called on the authorities to advance their efforts to arrest the offenders, probably part of the Boko Haram terrorist group, bring them to justice, and put an end to the activities of the group which threatens the peace and coexistence in the country."

State of emergency

Boko Haram, whose name means "Western education is forbidden," seeks to overthrow the Nigerian government and replace it with a regime based on Islamic law.

A state of emergency has been imposed in Borno state since May 14, including a complete communications blackout, as authorities seek to tackle the militant group.

Yobe and Adamawa states, which border Borno state, were placed under a state of emergency at the same time.

Declaring the state of emergency in May, Nigerian President Goodluck Jonathan blamed "terrorists" aimed at "rebellion" who have caused "fear among our citizens and a near breakdown of law and order in parts of the country."

He said the terrorists were mounting a "rebellion and insurgency" that threatened Nigeria's unity and stability, and pledged to send more troops to the three northeastern states.

"We have taken robust steps to unravel and address the root causes of these crises, but it would appear that there is a systematic effort by insurgents and terrorists to destabilize the Nigerian state and test our collective resolve," he said in a televised speech.

With an estimated 174.5 million people, Nigeria is Africa's most populous nation. The CIA World Factbook says the nation is roughly 50% Muslim and 40% Christian.

Violence between the mainly Muslim north and Christian south has taken at least 2,800 lives, according to Human Rights Watch.

Journalist Hassan John reported from Nigeria and CNN's Nima Elbagir from Nairobi, Kenya. CNN's Laura Smith-Spark, Nick Thompson and Mohammed Tawfeeq contributed to this report.