MAIDUGURI, Nigeria (Reuters) - Gunmen in Nigeria on Saturday released 27 teenage boys who were kidnapped from their school last week in the north-central state of Niger, while security forces continued to search for more than 300 schoolgirls abducted in a nearby state. Schools have become targets for mass kidnappings for ransom in northern Nigeria by armed groups. On Feb. 17, 27 students, three staff and 12 members of their families were abducted by an armed gang that stormed the Government Science secondary school in the Kagara district of Niger state, overwhelming the school’s security detail. One boy was killed during the raid. After their release, boys were seen by a Reuters witness walking with armed security through a dusty village, some struggling to stand and asking for water. A government official said the boys were aged between 15 and 18. The release comes just a day after the raid on a school in Zamfara state where gunmen seized 317 girls. Police on Saturday mounted a hunt for the girls, while parents waited in the school compound for news on their daughters. One of them, Lawal Muhammed, was hopeful his daughter would be released, saying the abductors wanted a ransom which could be paid. “These ones ... are already after ransom, so I know and believe that when the government settles with them, they will be able to release our daughters,” he told Reuters. School kidnappings in Nigeria were first carried out by jihadist groups Boko Haram and Islamic State West Africa Province, but the tactic has now been adopted by other militants whose agenda is unclear. In a statement late Friday, the presidency said President Muhammadu Buhari had urged state governments “to review their policy of rewarding bandits with money and vehicles, warning that the policy might boomerang disastrously.” The unrest has become a political problem for Buhari, a retired general and former military ruler who has faced mounting criticism over high profile attacks by the gangs known locally as “bandits”. Buhari replaced his long-standing military chiefs this month amid worsening violence. In December, gunmen raided a school in northwestern Katsina state and kidnapped nearly 350 boys, who were subsequently rescued by security forces. The highest profile school kidnapping was that of more than 270 schoolgirls abducted by Boko Haram from the town of Chibok in 2014. Around 100 of them remain missing.