A suspected female suicide bomber killed at least 12 people on Tuesday in Maiduguri, capital of Nigeria's Borno state, military and hospital sources said, three days after a multiple bomb attack in the city killed more than 50.
Maiduguri is the birthplace of radical Islamist Boko Haram insurgents who have been fighting for six years to revive a medieval Islamic caliphate in northeast Nigeria.
The militants tried to seize the city at the end of January, killing more than 100 people in the attack, and again in early February.
On Tuesday, a loud blast rocked Maiduguri just after 4pm (1500 GMT) and a military source at the scene said a woman had detonated her bomb at a roundabout near the Monday Market, which has been attacked numerous times before.
Passer-by Abdulaziz Olawale, who was a few metres (yards) away when the bomb went off, said he believed it had been the work of a female suicide bomber, and that he had seen her charred and dismembered body.
"The place was very busy. Many people were waiting to pick up taxis and some were walking down the road when the blast occurred," Olawale said.
A source at the State Specialist Hospital said 12 corpses had been brought to the morgue.
Another bomb was discovered and defused by police on Tuesday in Babalayi, a densely populated district of Maiduguri and about 500 meters from the scene of Thursday's explosion, a member of the civilian joint taskforce said.
A police spokesman did not immediately respond to requests for comment.
The city of about 2 million people was hit by several bombs on Saturday, one of them inside the Monday Market, in an attack that bore the hallmarks of Boko Haram.
Suspected Boko Haram militants also attacked the town of Ngamdu on the border of Yobe and Borno states on Tuesday, killing at least a dozen people.
The insurgency poses a major security threat to Nigeria as Africa's most populous country and biggest oil producer. Still, Boko Haram has lately been thrown onto the defensive after Chadian and Nigerien troops launched offensives in coordination with the Nigerian army, successfully reclaiming some important towns in Borno.
Nigerian government forces have also pushed the militants out of Adamawa state and regained ground in Yobe, although Boko Haram raids persist. Until recently, the insurgents controlled an expanse the size of Belgium.