Explosions were heard on Monday in Maiduguri, the Borno state capital, where Nigeria's homegrown jihadi Boko Haram extremists have waged an insurgency for more than a decade.
Emergency services told The Associated Press they could have been suicide bombings.
The blasts occurred at the entrance of the University of Maiduguri Teaching Hospital and two local markets, known as Post Office and the Monday Market, according to Sirajo Abdullahi, head of operations at Nigeria's National Emergency Management Agency, or Nema, in Maiduguri.
"There are casualties and they are still managing the causalities at the hospital," Abdullahi said.
"We can't give the actual figure until we count."
No group has yet claimed responsibility.
The Nigerian military said in a statement earlier it had repelled attacks by suspected Islamic militants in the early hours of Monday on the outskirts of Maiduguri.
For years, Nigeria has been battling a complex security crisis from different armed groups, especially in the northern part of the country.
Jihadi extremist groups, including Boko Haram and one of its factions, have been blamed for intensified attacks targeting Nigeria's military bases in the north-east of the country this month.
But attacks in Maiduguri in Borno state, which is the epicentre of Nigeria's 17-year struggle with extremist armed groups, have been rare in recent years after military operations.
Borno's governor Babagana Umara Zulum said in a statement Monday he condemned in the strongest terms the explosions.
"My thoughts and prayers are with the families of the victims and those injured as a result of the blast," Zulum said.
"The act is utterly condemnable, barbaric and inhumane."
He called on residents to remain calm, go about their usual activities and report any suspicious movement or activity to security agencies.
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