Three of the students kidnapped by gunmen in northern Nigeria earlier this week have been shot dead, a local official said Friday.

Criminals known locally as bandits attacked the private Greenfield University in Kaduna state on Tuesday evening, in the fifth known kidnapping assault on a school or college in the country since December.

"The armed bandits who kidnapped students of Greenfield University, have shot dead three of the abducted students," Samuel Aruwan, Kaduna state's internal security and home affairs commissioner, said in a statement. 

He said their remains had been found Friday close to the university. 

Two university officials told AFP that 20 students along with three non-academic staff were kidnapped but officials could not confirm those numbers. 

Bandits have ramped up kidnapping attacks in recent months, hoping to squeeze ransom payments out of the authorities.

Local authorities have said they would not pay any ransom. 

Recent mass kidnappings have prompted six northern states to shut public schools to prevent further attacks.