PARIS, Aug 14 — Six French humanitarian workers killed in Niger were likely the victims of a "premeditated” attack targeting Westerners, a source close to an investigation by French anti-terror prosectors said today.

"At this stage, there is no evidence that the attack specifically targeted (French NGO) Acted, although it cannot be excluded either,” said the source, who asked not to be named.

"On the other hand, it is an attack that appears to have been premeditated to target a priori mainly Westerners.”

Six French aid workers, their Nigerien guide and a driver were murdered on Sunday in the Koure National Park, a wildlife haven 60 kilometres (37 miles) from the capital Niamey.

The area is a popular a destination for weekend leisure trips by Niamey residents, including foreigners.

France’s anti-terror prosecutor’s office said Monday it would probe charges of murder "with links to a terrorist enterprise” and "criminal terrorist association” in relation to the killings.

A team of 11 specialised investigators departed France for Niger the following day.

No one has claimed responsibility for the attack carried out by gunmen on motorcycles.

"Given the modus operandi, the terrorist hypothesis is being favoured,” the source told AFP.

Suspicion has fallen on the Islamic State in the Great Sahara (ISGS), active in the shared border region of Mali, Niger and Burkina Faso, where it is being pursued by France’s Barkhane force fighting jihadists in the Sahel.

The French investigation will seek to determine whether the assailants had been tipped off bout the humanitarians’ visit to the national park.

A plane transporting the remains of the six French victims arrived at Orly airport south of Paris this morning from Niamey.

Prime Minister Jean Castex is to lead a private memorial service in the afternoon. — AFP