Fighters from the Islamist militant group Boko Haram have launched an attack on the key city of Maiduguri in north-eastern Nigeria, reports say.
Fierce fighting is said to be taking place on the outskirts of the city as militants attempt to enter it.
The military is carrying out air strikes, and a curfew has been imposed.
Maiduguri is home to tens of thousands of people who have fled Boko Haram attacks and was visited on Saturday by President Goodluck Jonathan.
Boko Haram, which means “Western education is forbidden” in the Hausa language, launched military operations in 2009 to create an Islamic state.
Thousands of people have been killed, mostly in north-eastern Nigeria.
‘Pray for us’
Residents of Maiduguri, the capital of Borno state, say they woke to sustained loud explosions and gunshots.
Roads and business have been closed by security forces.
The attack appeared to have begun in the Njimtilo district on the edge of the city.
One resident there, Rachel Adamu, told Reuters: “Please pray for us, we are in danger, under serious attack now.”
A resident of the Moronti area, Buba Kyari, told Agence France-Presse: “It is flying bullets everywhere. All we hear are sounds of guns and explosions. A rocket-propelled grenade hit and killed a person from my neighbourhood who was fleeing into the city.”
The BBC’s Chris Ewokor in Abuja says the military are carrying out co-ordinated air strikes and ground attacks against the insurgents.
Maiduguri would be a major prize for Boko Haram, which last tried to take the city in December 2013.