An American missionary has been released after spending more than nine months in captivity, the international Christian organisation he worked for has said.

Kevin Rideout - who was kidnapped outside his home in Niger's capital Niamey last year - was "in good health in the care of US officials", SIM International said in a statement on Friday.

The New York Times, which first reported Rideout's release, cited unnamed US officials saying that he was now in FBI custody, though details of how he was released, as well as who he had been held by, remained unclear.

There had been concerns that Rideout had been taken by militants, who have increasingly used violence and abductions as tactics.

Rideout's kidnapping in October was carried out by three armed men who took him to the western Tillaberi region, a location known as a hotbed of militant activity, news agency Reuters reported, citing diplomatic sources.

It was feared he may have been taken there by militants belonging to groups like Jama'at Nusrat al-Islam wal-Muslimin (JNIM), an al-Qaeda affiliate which has been linked to several attacks in the region.

Islamist militant groups operating from neighbouring Nigeria, Mali and Burkina Faso have plagued Niger for years, targeting both civilians and the military.

The region has seen a wave of violence as jihadists have been fighting government security forces as well as Western and other allied troops.

Within days of Rideout's kidnapping, the US warned its citizens against travelling to Niger due to an "increased risk of terrorism, crime and civil unrest".

The New York Times reported that Rideout, who is also a pilot, may have been transferred to another group prior to his release.

SIM International said he would soon be reunited with his wife, their children and his extended family.

Rideout had been working in Niger as a Christian missionary for 19 years and was also a pilot for SIM International, which operates in more than 70 countries.

The organisation said Rideout's release was "incredibly good news" after months of uncertainty about his wellbeing.

"Krista and the rest of Kevin's family are profoundly grateful for the prayers offered on their behalf by people around the world throughout his captivity," it said.

It added that the family had requested privacy while they "reconnect after this time apart".

Rideout's release comes as several countries in West Africa and the Sahel have been struggling with terror attacks.

Niger, Mali, Burkina Faso and Nigeria have been dealing with violence that has been going on for more than a decade - resulting in the loss of thousands of lives and causing regional destabilisation.

The region has also experienced a wave of military takeovers, with a string of democratic governments overthrown.