Nigeria's military says Islamic extremists are attacking Maiduguri, the biggest city in northeast Nigeria, and a soldier says dozens of combatants have been killed and wounded.

Troops are blocking roads leading to Maiduguri, a city of more than 1 million, preventing civilians from escaping.

Amnesty International says hundreds of thousands of civilians "are now at grave risk."

An officer involved in the fighting, who insisted on anonymity because he is not authorized to speak to the press, said the extremists attacked on three fronts around Maiduguri early Sunday and dozens of fighters have been killed.

Military spokesman Chris Olukolade tweeted that "coordinated air and land operations are being conducted now."

Maiduguri has been attacked many times in Nigeria's five-year Islamic insurgency that killed 10,000 people last year.