A total of eight people were killed, including six French nationals, in the attack near the town of Koure, which is an hour's drive from the capital Niamey.
The French nationals killed by gunmen in Niger were aid workers, Niger's defence minister has said.
"The six French citizens were NGO workers. We are still carrying out investigations," Defence Minister Issoufou Katambe told Reuters on Sunday.
Earlier, it was reported that six French tourists, their local guide and driver have been killed by unidentified gunmen riding motorcycles in an area of southwestern Niger home to the last herd of West African giraffes.
"There are eight dead: two Nigeriens including a guide and a driver, while the other six are French," Tillaberi Region governor Tidjani Ibrahim Katiella said on Sunday.
"We are managing the situation, we will give more information later," the governor said, without indicating who was behind the attack.
#BREAKING #Niger | On August 9, 8 people, including 6 French tourists and 2 Nigeriens, were killed after armed men carried out an attack at the Koure Giraffe Reserve in the Kouré area, Tillabéri. #terrorism pic.twitter.com/kiYkL4CBa0— Horizon Intelligence (@_hozint) August 9, 2020
A source close to the environmental services said the assault took place around 11.30am (1030 GMT) six kilometres (four miles) east of the town of Koure, 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.
"We do not know the identity of the attackers but they came on motorcycles through the bush and waited for the arrival of the tourists."
The source added that the tourists' vehicle belonged to the French humanitarian organisation ACTED.
Around 20 years ago, a small herd of West African or Niger giraffes, a subspecies distinguished by its lighter colour, found a safe haven from poachers and predators in the Koure region.
Today they are a key tourist attraction in the former French colony, enjoying the protection of local people and conservation groups.