The bodies of 11 Nigerien soldiers missing since Tuesday’s ambush have been discovered, bringing the death toll to 28.
It was initially reported that 17 soldiers were killed in the ambush, which happened at the same place four US soldiers were killed in 2017.
Niger and other countries in the Sahel have been facing a growing militant threat from several Islamist groups.
The Islamic State group has said it was behind the ambush but there has been no confirmation of this claim by the authorities.
Militants belonging to affiliates of al-Qaeda are also active in the region.
They are most active in neighbouring Mali, where French troops intervened in 2013 to prevent them from advancing on the capital, but they often stage cross-border raids.
The soldiers were in pursuit of militants who attacked a high security prison 50km (30 miles) on Tuesday outside the capital, Niamey, a government spokesman told the BBC.
“One of the soldiers’ vehicles drove into a landmine and then the assailants started firing at our soldiers,” Mr Zakariyyah told the BBC’s Focus on Africa programme on Tuesday.