The National Anti-Terrorism Prosecutor's Office (PNAT) said the investigation for "assassinations in connection with a terrorist enterprise" and "terrorist criminal association" will be carried out by the General Directorate for Internal Security (DGSI), the country's intelligence agency.
Eight people were killed by gunmen on Sunday in a Niger giraffe park in the Kouré area, south-east of the capital city of Niamey.
Six of them were French aid workers for the Paris-based ACTED and Geneva-based IMPACT Initiatives NGOs. The other victims were their driver a tour guide, both Niger nationals.
The two NGOs said in a joint statement that their seven colleagues were "senselessly and cowardly murdered".
"ACTED and IMPACT Initiatives condemn in the strongest terms the senseless and barbaric killing of our colleagues and their guide. At this tragic moment, our thoughts are with their families and loved ones at home and in Niger. All of our teams across the world join in their grief."
"This heinous crime must not go unpunished, nor will it distract us from our commitment to support the people of Niger," they added.
France's Foreign Minister Jean-Yves Le Drian sent his condolences to the victims' relatives on Monday and said that "those responsible for this heinous attack will have to answer for their actions".
"France is determined to ensure that the imperative to protect humanitarian and health personnel is respected," he went on.
The two countries' leaders, France's Emmanuel Macron and Niger's Issoufou Mahamadou, have also condemned the attack and discussed the issue in a phone call on Sunday.
Macron said that for the two countries "the determination to combat terrorist armed groups remains unchanged."
"The fight continues," he added on Twitter.
There are currently 5,100 French soldiers deployed across the Sahel region as part of Operation Barkhane, launched in 2014.
Their mission is to assist five African countries -- Burkina Faso, Chad, Mali, Mauritania, and Niger -- in their fight against armed groups including the Islamic State in the Greater Sahara.