NIAMEY: The death toll from an ambush on Nigerien troops near the Mali border has risen to 28, an army spokesman said on Thursday, a major setback for military operations trying to restore order in a region plagued by jihadist groups and allied criminal gangs.

Tuesday's ambush occurred in a region near the town of Tongo Tongo, where fighters from an Islamic State affiliate killed four US special forces and four Nigerien soldiers in an ambush in October 2017.

The spokesman did not say who was behind the attack, and there was no immediate claim of responsibility.

Despite years of heavy deployments of French, US and UN forces, Africa's Sahel region remains a tinderbox of Islamist fighters, ethnic militias and criminal smuggling rackets.

The border areas where Niger, Burkina Faso and Mali meet are especially dangerous and violence is worsening across the region. Gunmen suspected to be Islamists killed at least 10 in apparently sectarian attacks on churches in neighbouring Burkina Faso this week.