Troops have been deployed to northwest Nigeria, where heavily-armed criminals known as bandits terrorise communities, forcing more than 70,000 to flee into southern Niger, according to UN figures.

On Friday, seven children were killed and five injured by a Nigerian air strike, according to a local governor in Niger.

"There was a mistake with the Nigerian strikes on the border that resulted in victims on our territory in the village of Nachade," said Chaibou Aboubacar, the governor of Maradi region.

"The victims are 12 children, seven of them dead and five wounded."

Nigeria has neither confirmed nor denied launching an air strike in that area but a military spokesman said a probe was underway.

"We have seen the reports. An investigation is ongoing," Director of Defence Information Major General Ohwonigho Jimmy Akpor told AFP, without giving further details.

Aboubacar said four children were killed instantly and three died on the way to hospital.

He said he visited the children's graves on Saturday as well as the site of the air strike.

Doctors Without Borders (MSF), which works at nearby Madarounfa district hospital, gave a higher death toll of four children and eight adults, citing survivors.

The charity treated seven children, it said in a statement on Sunday.

"One of these children died shortly after arrival... Two others died after being transferred to the regional hospital in Maradi. A fourth child, aged 20 months, died in the bombing, according to survivors," MSF said.

"Six other people, adults, reportedly died at the scene," MSF added, "and two more died after being transferred to the hospital in Maradi."

Niger has since 2018 reinforced military patrols along its border with Nigeria to prevent incursions by "bandits," who are notorious for killing and kidnapping traders and stealing cattle.

Nigeria officially labelled the gangs terrorists earlier this year, a move that analysts say seems aimed at unlocking more resources for the military in the northwest.

"Bandits" made international headlines last year after kidnapping hundreds of students in a series of mass abductions from schools and colleges.

Students are often quickly released after ransom payments but many are still missing.

Niger and Nigeria are also fighting jihadist insurgencies.