Boko Haram reportedly want to make Maiduguri the capital of a breakaway Islamist state. Capturing the city, where insurgencies sprang from five years ago, would mark a major victory for the extremist group.
The group currently controls mostly rural areas along the Cameroon and Chad borders that make up a territory the size of Belgium.
Picture: People flee the violence in northeast Nigeria
Boko Haram are blamed for killing thousands of people, mostly in north-eastern Nigeria. The 200 Chibok schoolgirls kidnapped by the group last year made headlines around the world, and are still missing.
Last month Boko Haram attacked the towns of Baga and Doron Baga. Satellite pictures released by Amnesty International showed the extent of destruction.
The government said 150 people had been killed in those attacks but local officials say the figure is far higher and some have put it as high as 2,000.
The US has urged Nigeria not to postpone its presidential elections – due to take place of 14 February – because of the violence.
President Jonathan Goodluck faces a battle to be re-elected against a backdrop of increasing violence and unrest in Nigeria.