It would investigate charges of “assassinations with links to a terrorist enterprise” and “criminal terrorist association”, the office said in a statement.
“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 AFP on Sunday.
France confirmed its nationals were among the dead, without giving a figure.
French aid group ACTED 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.
It is believed to be the first such attack on Westerners in the area, a popular tourist attraction in the former French colony thanks to its unique population of West African or Niger giraffes.
“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 (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.
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.