Kidnappers have released 53 people including women and children they seized on a bus in Nigeria, local authorities said, while dozens of others taken from a school in a separate incident are still missing.

Criminal gangs known locally as "bandits" in northwest and central Nigeria have scaled up attacks in recent years, kidnapping, raping and pillaging. A gang last week seized 53 people, including 20 women and nine children, who were traveling on a state-owned bus in Kundu village in Niger State.

"I was delighted to receive the 53... bus passengers who were abducted by armed bandits a week ago," the governor of Niger State, Abubakar Sani Bello, said in a tweet late Sunday.

It is unknown if a ransom was paid but state representatives have previously said they would not pay any.

"We went through one week of dialogue, consultations, hard work and sleepless nights because we had to secure their release within the shortest possible time," the governor's spokesperson, Mary Noel-Berje, said in a statement.

The freed bus passengers were receiving medical checkups before being reunited with their families, she added. In a separate incident, 42 people including 27 schoolboys were abducted from a school last week, and are still missing.

"The Students of the Government Science College Kagara, are still in the hands of their captives but that everything is being done to ensure their release," Noel-Berje said.

A plane that was scheduled to take part in a surveillance mission in connection with a possible rescue operation crashed on Sunday near Nigeria's capital Abuja, killing seven people. Gunmen last week killed 10 people and abducted at least 23 others in two separate attacks in the state.

"We are witnessing these attacks now, almost, on daily basis and it is worrisome," Noel-Berje said at the time. Bandits are known to hide in camps in Rugu forest, which straddles Zamfara, Katsina, Kaduna and Niger states. Despite the deployment of troops, deadly attacks persist.

The recent attacks have raised concern about rising violence by armed gangs fueling criticism of Nigeria's President Muhammadu Buhari's handling of national security. In January, the president appointed a new military high command.

Violence and insecurity have compounded the economic challenges faced by citizens in Africa's most populous country, which is struggling to cope with a fall in revenues due to an oil price slump on top of the COVID-19 pandemic.