Fifty of the more than 300 children snatched by gunmen from a Catholic school in Nigeria have escaped their captors, a Christian group said in a statement on Sunday.

"We have received some good news as 50 pupils escaped and have reunited with their parents," said the Christian Association of Nigeria (CAN) in a statement, adding they escaped between Friday and Saturday.

The schoolchildren are aged between 10 and 18.

Gunmen on Friday raided St. Mary's co-education school in Niger state, taking 303 children and 12 teachers in one of the largest mass kidnappings in Nigeria.

It was not immediately clear where the children were being held or how they managed to return home.

No group has yet claimed responsibility and authorities have said tactical squads have been deployed alongside local hunters to rescue the children.

"As much as we receive the return of these 50 children that escaped with some sigh of relief, I urge you all to continue in your prayers for the rescue and safe return of the remaining victims," CAN chairman in Niger State, Reverend Bulus Dauwa Yohanna, said in the statement.

The abduction came after gunmen had on Monday stormed a secondary school in neighbouring Kebbi state, abducting 25 girls, as security fears mount in Africa's most populous nation, sparking a wave of school closures across other parts of the country. (Agencies)