The more than 250 school students seized by gunmen in a mass kidnapping in northwestern Nigeria earlier this month have been released, the local governor said on Sunday.

The kidnapping in Kuriga, Kaduna state on March 7 was one of the biggest such attacks in years and prompted a national outcry over insecurity.

"The abducted Kuriga school children are released unharmed," Kaduna state governor Uba Sani said in a statement that did not specify how they were freed.

"This is indeed a day of joy," he said, thanking the army, President Bola Ahmed Tinubu, the national security adviser, and "all Nigerians who prayed fervently for the safe return of the school children".

Gangs of criminals known locally as bandits have been blamed for the abductions. They routinely target communities, loot villages and carry out mass kidnappings for ransom in northwest and north-central Nigeria.

Relatives had said the kidnappers demanded a large payment for the return of the students, but President Tinubu said he had ordered security forces not to pay up.

Kidnap victims in Nigeria are often freed following negotiations with the authorities, though a 2022 law banned handing over money to kidnappers and officials deny ransom payments are made.