GUSAU, Nigeria (Reuters) - Gunmen have released girls kidnapped from a boarding school in northwest Nigeria, the governor of Zamfara state said in a message posted on Twitter on Tuesday in which he shared pictures girls wearing Muslim veils.

An armed gang abducted 317 girls from the Government Girls Science Secondary (GGSS) School in the town of Jangebe at around 1 a.m. on Friday. The governor’s tweets did not state how many girls had been released, but carried images of girls.

“Alhamdulillah! It gladdens my heart to announce the release of the abducted students of GGSS Jangebe from captivity”, Zamfara State Governor Bello Matawalle said on Twitter.

“This follows the scaling of several hurdles laid against our efforts. I enjoin all well-meaning Nigerians to rejoice with us as our daughters are now safe,” he said.

Schools have become targets for mass kidnappings for ransom in northern Nigeria by armed groups, in a trend started by the jihadist group Boko Haram, and later its offshoot Islamic State West Africa Province. Criminal gangs have carried out kidnappings since.

The government has repeatedly denied paying ransoms. But President Muhammadu Buhari issued a statement on Friday in which he urged state governments “to review their policy of rewarding bandits with money and vehicles, warning that the policy might boomerang disastrously”.

The raid in Zamfara state was the second such kidnapping in little over a week in the northwest, a region increasingly targeted by criminal gangs. On Saturday, gunmen released 27 teenage boys who were kidnapped from their school on Feb. 17 in the north-central state of Niger.

Nigeria’s most high profile school kidnapping was that of more than 270 schoolgirls abducted by Boko Haram from the northeastern town of Chibok in 2014. Around 100 of them remain missing.