The Nigerian army said on Thursday that its troops had freed seven children and two adults who had remained in captivity following a kidnapping from an orphanage last month.

Mass kidnappings carried out by gangs and armed groups has been a persistent challenge in Africa's most populous country, especially in rural areas.

In late April, gunmen raided an unregistered orphanage in central Kogi state, seizing 23 children and several adults.

Most of the children were freed shortly after, but seven remained missing.

Nigerian army troops "successfully rescued the remaining kidnapped victims" after "sustained search and rescue efforts", the army said in a statement.

"The rescued victims comprised five boys, two girls and two adult females, believed to be the wives of the proprietor of the orphanage," it said.

‘Operating illegally’

Kogi's information commissioner Kingsley Fanwo had previously said that the orphanage "was operating illegally in a remote, bushy environment without registration with the state government and without the knowledge of relevant authorities and security agencies".

Nigeria faces multiple security threats, including terrorism, "bandit" gangs and farmer-herder violence.

North-central Nigeria, where Kogi is located, has seen violent attacks including raids on schools in recent months, with some blamed on jihadist groups.

Hundreds of children were taken by gunmen from their school in neighbouring Niger state in November, in an attack security sources blamed on Boko Haram terrorists.