The police commissioner put the number of dead at 34.
It was not clear who was behind the attack on Wednesday night in the town of Potiskum, in Yobe state, which has been an occasional target for militant Islamist sect Boko Haram.
“I have counted 56 bodies at the morgue and I am sure that the death toll could rise in view of the serious nature of injuries sustained,” the nurse at Potiskum hospital, who gave his name as Babangida, said.
“The Potiskum mortuary is made up of a room and a parlour and I counted the 56 in the parlour only. I didn’t go into the inner room.”
Police Commissioner Moses Namiri said security forces had confirmed 34 killed and that Islamist sect Boko Haram was suspected to be behind the attack.
“Everybody knows the modus operandi of BH (Boko Haram): they threw explosives and used guns,” he said. “The gunfire lasted for almost an hour.”
Witness Mama Yusuf, a retired civil servant, said there were dead bodies on the ground, though he could not say how many.
“I saw dead bodies all around the place and the emergency services taking people to hospitals,” he said.
Boko Haram has been fighting a low-level insurgency for more than two years and has become the main security threat facing Africa’s top oil producer, although it is far from any oil producing facilities.
It usually targets police or authority figures, and although civilians have increasingly borne the brunt of its attacks, they are normally targeted for being a perceived enemy of the group, such as Christians, rather than being randomly killed.
Sometimes violence in Nigeria, especially in parts of the north or the volatile Middle Belt – where the largely Christian south and Muslim north meet – is driven by ethnic rivalry over land and resources that has little to do with the Boko Haram attacks.
Boko Haram, which wants to impose an Islamic state on Nigeria’s mixed population of Muslims and Christians, has been blamed for hundreds of killings since its uprising against the government began in 2009.
A spate of attacks in the past few days, including one against Christians in the north that killed 19 people on Sunday, have dampened hopes that tighter security had significantly reduced the sect’s capability.
Nigerian forces killed the suspected mastermind of Sunday’s attack on Christian worshippers, in a raid in the main northern city of Kano on Tuesday that resulted in a gun battle lasting several hours.