Two female suicide bombers have blown themselves up at a crowded market in the Borno State capital, Maiduguri, killing at least 30 people, witnesses say.
One teenage girl blew herself up first and as people gathered to help victims, the second girl detonated herself, the witnesses added.
One man told the BBC he heard loud sounds as the bombs exploded.
Militant Islamist group Boko Haram is waging an insurgency in Nigeria.
It was based in Maiduguri, after its formation in 2002, but it has since been driven out of the city by the military and vigilante groups.
It now controls a large number of towns and villages in Borno, amid fears that it is preparing to launch an assault to capture Maiduguri.
Boko Haram has not commented on the explosions.
It has been blamed for at least five female suicide bombings in Nigeria since June.
The girls, dressed in full hijab, entered the busy market and detonated their explosives, said Abba Aji Kalli, the coordinator of a government-backed vigilante group in Borno, AP news agency reports.
The first bomber killed about three women, he said.
When people gathered at the scene, the second bomber screamed and blew herself up, killing about 30, Kalli added.
A hospital source confirmed that at least 30 people had been killed, the BBC’s Mansur Liman reports from the capital, Abuja.
Borno has been the epicentre of Boko Haram’s campaign.
On Monday, suspected insurgents took over Damasak town, less than 200km (125 miles) from Maiduguri, setting fire to houses and killing an unspecified number of people.
The assailants entered the town disguised as traders, carrying boxes which were packed with AK-47 rifles.
The insurgency has intensified despite the government declaring a state of emergency last year in Borno and two neighbouring states.