A car bomb has exploded in the north-eastern Nigerian city of Maiduguri, killing at least 17 people.
The bomb went off near a market, sending up a large plume of smoke. People were seen fleeing the scene covered in blood.
Boko Haram has been conducting a four-year campaign of violence to push for Islamic rule in northern Nigeria.
The immediate aftermath of Tuesday's blast was described as chaotic, with bodies on the ground and troops firing automatic weapons.
One witness told the BBC: "I heard a very loud explosion followed by several other explosions. After a while, I came out of my shop and saw many lifeless bodies lying on the floor."
Maiduguri, the capital of Borno state, was considered the epicentre of activity by Boko Haram, which translates as "Western education is sin". But members of the group were forced out of the city by a combination of security forces and a vigilante group known as civilian JTF following the imposition of emergency rule in May last year, BBC Hausa Editor Mansur Liman reports.
This is the latest in a string of attacks attributed to Boko Haram. On Sunday, militants are reported to have attacked a market in the village of Kayamula, also in Borno state. Five traders were killed.
Boko Haram was also suspected of being behind an attack on 20 December on a military barracks in the town of Bama, south-east of Maiduguri.
The group carried out co-ordinated attacks on Maiduguri's air base and military barracks - also in December - causing extensive damage.
The state of emergency declared in May was imposed in Borno and the neighbouring states of Adamawa and Yobe.
United Nations figures suggest more than 1,200 people have been killed in Islamist-related violence since the state of emergency started.