Gunmen on Sunday killed eight people including a group of French aid workers as they visited a part of Niger popular with tourists for its wildlife.
"There are eight dead: two Nigeriens including a guide and a driver, while the other six are French," the governor of the Tillaberi region told the French AFP news agency on Sunday.
France confirmed its nationals were among the dead, without giving a figure.
French aid group Acted (the Agency for Technical Development and Cooperation) said several of its workers were among those killed during a tourist outing.
"Among the eight people killed in Niger, several were Acted employees," said the NGO's lawyer Joseph Breham.
Identity of attackers remains unknown
"We are managing the situation, we will give more information later," governor Tidjani Ibrahim Katiella said, without indicating who was behind the attack.
A source close to Niger's environmental services said the assault took place at around 11:30 am, six kilometres east of the town of Kouré, which is an hour's drive from the capital Niamey.
"Most of the victims were shot... We found a magazine emptied of its cartridges at the scene," the source told AFP.
One woman managed to escape but was later caught and killed, the source added.
"We do not know the identity of the attackers but they came on motorcycles through the bush and waited for the arrival" of the group.
The source said the victims' vehicle belonged to Acted, which was later confirmed by Niger's home ministry.
Wide condemnation of a barbarous act
The office of French President Emmanuel Macron said he spoke on the phone with his Niger counterpart Mahamadou Issoufou.
Both leaders condemned the "cowardly" attack.
In Paris, a spokesman for the French army said France's Barkhane force, which fights jihadists in the Sahel region, had provided support to Niger's forces.
An AFP reporter at the scene confirmed that French fighter jets flew overhead later Sunday as Niger's army searched the vast wooded area.
Forensic police were collecting evidence at the scene.
IBK laments violent extremism
Neighbouring Mali's President Ibrahim Boubacar Keita meanwhile strongly condemned the "barbaric act".
He lamented that "violent extremism" was still rife in the Sahel region "despite the merciless war waged by national armies, the G5 Sahel joint forces and the Barkhane force".