Suspected Boko Haram militants are attacking Nigeria's northern city of Maiduguri in Borno state from a cashew plantation a few kilometers from the Giwa barracks, a military source said.

Residents said they heard heavy shooting on the outskirts.

"I'm trapped near the University of Maiduguri now," local resident Kabir Olaoye told Reuters by phone. The shooting began around 6:45 pm (1745 GMT) local time, he said.

The city of around 2 million people is the birthplace of the Islamist jihadi group, which has killed thousands in its attempt to carve out an Islamist state in the country's northeast.