RNA - The blast happened 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, said a local resident and a lecturer, New Arab reported.
There was no immediate claim of responsibility but suspicion will likely fall on the terror group Boko Haram, which has repeatedly used young women and girls as human bombs.
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 5:15 am.
"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 University of Maiduguri Teaching Hospital."
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 in to and out of the city, is vast and most areas are badly lit.
Students also say security has been relaxed in recent months and some of the soldiers who manned the campus gates have been withdrawn.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 insurgency is in its final stages.
Last month 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 targeted a base in Buni Yadi, in neighbouring Yobe state.
The following day, suicide bombers killed three in Maiduguri.
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