image sourceAFPimage captionThe children were taken from an Islamic school in Tegina, Niger state

Gunmen in Nigeria have freed a number of pupils who were kidnapped from an Islamic school in May, according to their head teacher.

Some 136 students from the school in Tegina, Niger state, were seized by gunmen demanding a ransom.

School officials say 15 students escaped in June and a further six died while in captivity.

Mass abductions for ransom have become increasingly common across Nigeria in recent months.

Head teacher Abubakar Alhassan said he could not give an exact number of how many students have been freed, but "none of the pupils are in captivity".

He told AFP news agency that the students were being transported home.

It is not yet clear how the pupils were released.

Fati Abdullahi, whose 18-year-old daughter and 15-year-old son were taken by the group, said her children had been freed.

"We are anxious to see them," she said.

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On 30 May, gunmen riding on motorcycles stormed the town and opened fire, killing one person and injuring another.

As people fled, the attackers went to the school and seized the children.

In July, kidnappers seized a man who was sent to deliver a ransom payment to secure the students' release. Parents and school administrators had sold possessions and part of the school's land to pay the ransom. But they later said the payment was not enough.

More than 1,000 students have been abducted from schools across northern Nigeria since December last year.

Authorities there have been criticised for their failure to tackle the country's widespread insecurity including the deepening kidnapping crisis.