Troops in Niger backed by French air power launched a hunt Monday for the killers of eight people, six of them French nationals, as prosecutors in Paris announced a terror investigation into their deaths.
"Search operations with our partners are under way to root out the people behind these vile acts and strengthen security in the area," the Nigerien interior ministry said.
The French military, which has a 5,100-member anti-jihadist force in the Sahel, said it was providing air support for the operation, which is unfolding over a vast wooded area.
Six French citizens were killed on Sunday along with their Nigerien guide and driver about six kilometres (four miles) from the town of Koure, in a wildlife haven about an hour's drive southeast from the capital Niamey, officials said.
The country, one of the poorest in the world, is struggling with incursions by Islamists from both Nigeria to the south and Mali to the west.
In Paris, French anti-terror prosecutors said in a statement they would investigate charges of "murders with links to a terrorist enterprise" and "criminal terrorist association".
French NGO Acted, whose staff were targeted in the attack, said the victims were aged between 25 and 50 and were four women and four men. It did not give their names.
Acted co-founder Frederic Roussel told reporters in Paris that the international community had to do more to ensure the protection of humanitarian workers.
"The international community must understand the contradiction between asking us to support these populations who live under the most dramatic circumstances and leaving us alone against violence where we are the easiest targets," he said.
- 'Cowardly and barbaric' -
The attackers came on motorcycles through the bush and waited for the group's arrival, a source said.
Most of the victims were shot although one woman managed to escape but was later caught and her throat was cut, the source added.
The bodies were laid side-by-side next to a torched 4x4, which had bullet holes in its rear window.
Nigerien forensic experts investigated the site of the killing before the bodies were taken to Niamey.
French President Emmanuel Macron and his Nigerien and Malian counterparts Mahamadou Issoufou and Ibrahim Boubacar Keita lashed the attack as cowardly and barbaric.
The Koure area is a sanctuary for hundreds of West African giraffes, a subspecies distinguished by its lighter colour.
"We all go to Koure on weekend outings because it's very easy to access," a Western humanitarian source based in Niamey told AFP.
"Everyone goes there, even ambassadors, diplomats, teachers... it is not considered a dangerous zone at all."
Later on Monday, officials said they had suspended access to the park and extended a pre-existing state of emergency to cover the Kollo prefecture, which houses the giraffe park.
The region around Niamey has become a hideout for jihadist groups including the Islamic State in the Greater Sahara (ISGS), which have killed hundreds of troops and driven thousands of people from their homes.
- Previous kidnappings -
Nigerien authorities banned humanitarian organisations from travelling without military escort in several areas of Tillaberi and the neighbouring region of Tahoua in 2019 after armed men stole vehicles belonging to aid groups.
Two humanitarian workers, an American and a German, had been kidnapped in those regions in 2016 and 2018.
Aid groups said Monday they would not be deterred by the deteriorating security situation.
"The UNHCR, like all humanitarian actors in Niger, will continue its mission to help the most fragile populations," said Alessandra Morelli, the representative in Niger for the UN refugee agency UNHCR.
"We try to be close to the people, we try to go out into the field to assist these populations affected by the violence, but we notify the relevant authorities of our movements," Nicolas von Arx, head of the International Committee of the Red Cross (ICRC) delegation in Niger, told AFP.