Scores of people were killed in a bombing at a market in Maiduguri, the capital of the Nigerian restive state of Borno.
Eyewitnesses told AP at least 20 people died in the blast, which was reportedly caused by a suicide attacker.
But hospital sources speaking to Reuters put the death toll at more than 50, with the news agency saying the explosion came from a bomb concealed under a butcher's stand.
No group has claimed responsibility for the attack but suspicion is likely to fall on Islamist group Boko Haram (now Iswap), which has recently resumed attacks in Maiduguri and other towns in north-eastern Nigeria.
Borno, Yobe and Adamawa are the Nigerian states that mostly bear the brunt of Boko Haram's Islamic insurgency that killed 10,000 people in 2014 alone.
Maiduguri, the biggest city in north-east Nigeria, has been attacked many times over the past six years.
The latest bombing came days after newly elected President Muhammadu Buhari announced he would relocate military headquarters from Abuja to Maiduguri.
Meanwhile, Boko Haram released a graphic video purportedly showing one of its fighters executing wounded soldiers with a shot in the head.
The Nigerian government is currently aided by mercenaries and troops from neighbouring countries Chad, Benin, Niger and Cameroon in its offensive and has scored some successes since the military cooperation started in February.
Earlier in May, the army announced it had entered what has been deemed as "the terrorists' last-known stronghold" in the Sambisa forest, on the border with Cameroon, and freed nearly 700 people captured by the group.
Buhari, a former military general and member of the All Progressive Congress (APC) party, vowed to end Boko Haram's insurgence and said the country would do everything it could to find the 220 girls abducted by the insurgents from Borno's Chibok village in April 2014.
The US announced it would send military trainers to help the Nigeria army improve its intelligence gathering and logistic.