Since May 1, when two French tourists went missing in a wildlife park in neighbouring Benin, French soldiers and special forces from the regional Barkhane force had swung into action in the hunt for their kidnappers.

On May 7, a first special forces mission was launched inside Burkina Faso which returned vital information on the gang of six hostage-takers who were thought to be heading for lawless Mali to the northwest, French chief of staff Francois Lecointre said.

As the group travelled across the country -- in view of drones known to be operated by French and American forces in the area -- the French waited for them to stop and an opportunity to intervene.

In the meantime, specialised battlefield medical teams were flown from Paris to be moved in by helicopter when the final order came to launch an assault.

"It was an extremely complex operation, with extremely demanding timings," Lecointre told reporters in a detailed briefing on the operation in Paris with Defence Minister Florence Parly.

French military commanders judged that Thursday night's stop in northern Burkina by the gang was the last opportunity to intervene before the hostages were taken into Mali and transferred to an Islamist militant group there.

"It was the last opportunity to carry out an operation in Burkina Faso. If the hostages had been transferred to Mali, an operation like this would have been too risky," Parly said.

"It was important to act quickly. I continue to think it was the right decision to do it," she added.

- Surprise discovery -

French President Emmanuel Macron, who was in Romania for a meeting with fellow European leaders, was informed and approved the intervention on Thursday evening, making use of a recently-concluded military cooperation deal with Burkina Faso.

Around 20 French commandos were then dropped by helicopter at distance from the camp where the hostage-takers and their victims were thought to be sleeping.

Approaching in silence under the cover of darkness, the teams got past a guard and to within a few metres of the four shelters where the hostages were being held before they were finally detected, Lecointre said.

"The commandos went inside the shelters without opening fire," he added, underlining the difficulty of hostage situations even for highly-trained special forces who have to be careful not to harm the people they are rescuing.

Two French soldiers died in shots fired at close range by the hostage-takers.

Four of the gang were killed in return fire and two escaped.

To their surprise, instead of finding just two French male tourists, the rescue teams discovered two women as well -- an American and a South Korean.

"No one was aware of their presence," Parly explained, while Lecointre said they had "apparently (been held) for 28 days".

"The contacts we have had in recent hours with the United States and South Korea indicate that these countries were probably not aware of the presence of their nationals on Burkina territory," Parly explained.

She thanked authorities in Benin and Burkina Faso for their help with the "complex operation", as well as the United States which provided intelligence and support.