Nigerian gunmen have freed dozens of schoolchildren kidnapped earlier this month in northwest Zamfara State, according to a local government source and a video showing state officials with the children.

The release of the Kaya school students on Sunday came after the army began a crackdown on criminal gangs in the state and local authorities shut down telecoms in Zamfara to disrupt communications between armed groups.

More than 70 students and some teachers were snatched in Kaya on September 1 in the latest in a series of mass abductions at schools and colleges this year by heavily armed gunmen known locally as bandits.

"A total of 75 hostages taken from the Government Junior Secondary School Kaya were released on Sunday evening," the local government source said. "They looked robust and unharmed."

A video released by Zamfara State Governor Bello Matawalle's office showed him greeting buses full of students in the night and asking them if they had been harmed.

According to security sources and the local source, their captors had released them in exchange for safe passage out of the forest as the army had surrounded their camp.

With security operations ongoing in the state, gunmen also attacked a military base in Zamfara on Saturday, killing 12 security forces members before stealing weapons and torching building, two security sources said Monday.  

It was not immediately clear who carried out the raid in Mutumji, but the base about 80 kilometres (50 miles) from the state capital Gusau, is a key site for logistics and reconnaissance in the army's fight against bandits in the area.

Jihadists in Nigeria's northeast frequently attack military bases in Borno State, the heart of a 12-year insurgency that has killed 40,000 people.

But the large, heavily armed criminal gangs who raid villages and kidnap for ransom in the northwest and central states have also become more brazen.

In July, bandit gunmen shot down an air force jet over Zamfara while it was returning from operations. The pilot safely ejected and managed to escape capture.

Gunmen also attacked the country's elite defence academy in northwest Kaduna State last month, killing two officers and kidnapping another in a raid in one of the symbols of the armed forces.  

Since December the bandit gangs have also increasingly targeted schools for abductions, and more than 1,000 students have been kidnapped over this year in a string of raids.

Most of those students have been released after negotiations and some escaped, but dozens are still being held in forest hideouts by their bandit captors.