This image taken from TV, shows the interior of a mosque after a deadly attack by a suicide bomber, in Mubi, Adamawa State, Nigeria, Tuesday Nov. 21, 2017. A teenage suicide bomber detonated as worshippers gathered for morning prayers at a mosque in northeastern Nigeria, killing at least 50 people, police said Tuesday, in one of the region's deadliest attacks in years. (AP Photo)

This image taken from TV, shows the interior of a mosque after a deadly attack by a suicide bomber, in Mubi, Adamawa State, Nigeria, Tuesday Nov. 21, 2017. A teenage suicide bomber detonated as worshippers gathered for morning prayers at a mosque in northeastern Nigeria, killing at least 50 people, police said Tuesday, in one of the region's deadliest attacks in years. (AP Photo)

LAGOS, Nigeria (AP) — A teenage suicide bomber attacked worshippers as they gathered for morning prayers Tuesday at a mosque in northeastern Nigeria, killing at least 50 people, police said, in one of the region’s deadliest assaults in years.

Bloody debris covered the floor inside the mosque in Mubi town in Adamawa state where worshippers had arrived around 5 a.m. Outside, people gathered around the dead.

President Muhammadu Buhari tweeted that he was “saddened by the very cruel and dastardly suicide bombing attack.”

“May the souls of the dead rest in peace,” he added.

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Police spokesman Othman Abubakar told The Associated Press they were “still trying to ascertain the number of injured because they are in various hospitals.”

While there was no immediate claim of responsibility for the bombing, suspicion immediately fell on the Boko Haram extremist group. The group is based in neighboring Borno state and has been blamed for scores of similar attacks over the years.

Tuesday’s attack was the first since Mubi town was liberated from Boko Haram insurgents in 2014.

Boko Haram increasingly has been using teenagers or young women as bombers, many of whom have been abducted.

The police spokesman said the young man detonated his explosives Tuesday while mingling among the worshippers.

While Nigeria’s military in recent months has flushed Boko Haram from its forest stronghold, President Muhammadu Buhari’s claim late last year that the extremist group had been “crushed” has proven to be premature.

Boko Haram has been blamed for more than 20,000 deaths during its eight-year-old insurgency. The attacks have spilled into neighboring countries and displaced more than 2.4 million people in the Lake Chad region, creating a vast humanitarian crisis.

U.N. Secretary-General Antonio Guterres condemned the suicide attack and reiterated “the solidarity of the United Nations with the Government of Nigeria in its fight against terrorism and violent extremism,” U.N. deputy spokesman Farhan Haq said.

The U.N. chief called for those responsible “for these heinous acts to be swiftly brought to justice,” he said.

The United States condemned the attack. A statement by U.S. State Department spokeswoman Heather Nauert said “that the victims were targeted and killed in a place of worship demonstrates yet again the brutal nature of the terrorists whose sole aim is to threaten the peace and security of Nigerian citizens.”

Nauert said the attack only strengthens the resolve of the U.S. to work with Nigerian and regional partners in countering such threats.

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Associated Press writer Ibrahim Abdulaziz in Yola, Nigeria, contributed to this report.