French prosecutors on Monday said they were launching an investigation into "terrorist murder" of at least six humanitarian workers in Niger.

The aid workers and their local guide were shot dead by gunmen on Sunday in southwestern Niger. It was initially thought the gunmen targeted tourists.

The workers were associated with Paris-based NGO ACTED and Geneva-based IMPACT Initiatives. The NGOs condemned "in the strongest terms the senseless and barbaric killing of our colleagues and their guide."

"We want to protect the victims' anonymity for now, but we can say they were aged 25 to 50 and included four women and four men," ACTED development director Frederic Roussel told a news conference in Paris on Monday.

French NGO ACTED has been working in Niger since 2010 and provides aid to displaced people and local populations that are vulnerable due to conflicts in the region, lack of food and droughts. IMPACT Initiatives, which is a partner to ACTED, was first deployed in Niger in 2012. It conducts mapping programs and other projects in camps hosting displaced populations.

"Our colleagues have been working to support the people of Niger facing hardship, driven by values of humanity and solidarity," the NGOs said in a statement.

No group has so far claimed responsibility for the assault.

Read more: Niger: Attack on military camp kills dozens of soldiers

'Cowardly' attack

Sources say the attack took place around 11:30 a.m. (1030 GMT) on Sunday, 6 kilometers (about 4 miles) east of Koure, which is an hour's drive from the capital, Niamey.

The area where the attack occurred is home to the last West African giraffes. The Koure Giraffe Reserve southeast of Niamey is a popular tourist attraction in Niger, a country that borders seven countries in an unstable region including Libya, Mali, Chad and Nigeria.

The Association of Giraffe Guides of Koure condemned the death of its president, Kadri Abdou, in the attack.

"We are deeply saddened and thinking of the victims and their families to whom we offer our most sincere condolences and especially to the family of Kadri, our friend. May he rest in peace,'' said the statement.

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The Elysee Palace confirmed that French citizens were killed in the attack. President Emmanuel Macron expressed his condolences for the victims in a statement, denouncing the violence as "cowardly." Macron also spoke to Nigerien President Mahamadou Issoufou about the attack. In a tweet published on Sunday, Issoufou condemned "the cowardly and barbarous terrorist attack perpetrated in the peaceful area of Koure."

French Foreign Affairs minister Jean-Yves Le Drian vowed that those responsible [for the attack] "will have to answer for their acts, adding that "the ministry's crisis center and the French embassy in Niger are fully mobilized to support the victims' families."

Jihadi activities in the region

French authorities regularly warn citizens against travelling to most parts of Niger, where militant groups, including Boko Haram and affiliates of the "Islamic State" (IS) group are active. The West African country's Kouré area, however, is considered relatively safe.

In January, Nigerien authorities banned the use of motorcycles in an attempt to curb the movements of jihadis.

France, a former colonial power in the region, launched a coalition of West African and European allies in June to fight Islamist militants in the Sahel region, which includes Niger.

Read more: Anti-French sentiment on the rise in West Africa as security situation deteriorates

In October 2017, militants linked to IS killed four US soldiers in an ambush in Niger.

Read more: US troop drawdown from Sahel puts pressure on Europe

shs/rc (Reuters, dpa, AFP, AP)