At least four people were killed and 15 others injured in a suicide bomb attack on a university campus in northeast Nigeria, police said.

The blast happened on Monday at a mosque in the staff quarters area of the University of Maiduguri, and is thought to have been carried out by a teenage girl, a local resident and a lecturer told AFP news agency.

There was no immediate claim of responsibility but suspicion will likely fall on the armed group Boko Haram, which has repeatedly used young women and girls as human bombs.

Al Jazeera's Ahmed Idris, reporting from the Nigerian capital Abuja, said the violence was likely the outcome of Boko Haram losing ground in its strongest hideout, the Sambisa Forest, in the northeast of the country, which is now under military control.

"The leader of the Boko Haram group faction, Abubakar Shekau, called on all his supporters to hit the Nigerian population as hard as possible through bombs, guns and even slash-and-burn," he said.

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Borno police spokesman Victor Isuku said that before the blast at the mosque, police on patrol shot a girl aged about 12, as she tried to get into the university at about 04:15 GMT.

"The IED [improvised explosive device] strapped to her body exploded, killing her instantly," he said in a statement.

"Shortly after that, a second explosion occurred in a mosque inside the university.

"Four persons, including a professor and the second suicide bomber, died, while 15 persons sustained various degrees of injuries and were rushed to UMTH [University of Maiduguri Teaching Hospital]."

One local resident, who asked not to be identified, said he heard two explosions as he was leaving the mosque in his neighbourhood.

"I rushed to the university, which is not far from my house. On entering, I saw the mosque in a mess. Three people lay dead, including a young girl, whose body was badly mutilated," he said.

"Thirteen other worshippers were injured."

A university lecturer who lives on site confirmed the resident's account.

Disruptions

Filibus Yamta Mshelia, from the registrar's office at the university, said examinations scheduled for Monday had been postponed to a later date because of the attack.

Examinations set for Tuesday will take place as planned, he added in a letter to staff and students.

"We urge all students to remain calm and vigilant," he wrote.

Access to the university campus has been strictly controlled, with checkpoints and searches at the gates which until now had prevented a direct attack.

But the site, on one of the main thoroughfares leading into and out of the city, is vast and most areas are badly lit.

Students also said security has been relaxed in recent months and some of the soldiers who manned the campus gates have been withdrawn.

At least 20,000 people have been killed in the Boko Haram conflict since it began in 2009 and more than 2.6 million others made homeless.

Nigeria maintains the armed rebellion is in its final stages.

Last month, the government said it had flushed out Boko Haram fighters from their stronghold in the Sambisa Forest area of Borno state, of which Maiduguri is the capital.

Yet there has been no let-up in attacks both on troops and civilians.

On January 8, at least five soldiers were killed when rebels attacked a base in Buni Yadi, in neighbouring Yobe state.

The following day, suicide bombers killed three in Maiduguri.