The operation was launched to free two French tourists who had disappeared while on holiday in the remote Pendjari National Park in neighbouring Benin on May 1.

But during the raid, the French troops were surprised to discover two women also in captivity, with top officials saying they had been held for 28 days.

The French tourists were identified as Patrick Picque, 51, and Laurent Lassimouillas, 46, but the women's identities were not immediately clear.

"No one was aware of (the women's) presence," French Defence Minister Florence Parly told reporters, while French armed forces chief Francois Lecointre said.

"We know little about these other two hostages," Parly told reporters, saying that even Seoul and Washington did not appear to be aware the pair were in increasingly unstable Burkina Faso.

The raid was approved by French President Emmanuel Macron in what was seen as the last opportunity to stop the hostages being transferred to lawless territory in Mali to the north.

Parly said it was "too early to say" who had snatched the two French nationals from Benin, which has long been an island of stability in a region where Islamist militants are increasingly active.

"The message to terrorists and criminal gangs is clear: those who attack France and its nationals know that we will not spare any effort to track them down, find them and neutralise them," she said.

Four of the six kidnappers were killed in the raid.

French forces, helped by intelligence provided by the United States, had been tracking the kidnappers for several days as they travelled across the semi-desert terrain of eastern Burkina Faso from Benin to Mali.

They seized the opportunity to prevent "the transfer of the hostages to another terrorist organisation in Mali," Lecointre said, referring to the Macina Liberation Front (FLM).

The FLM is a jihadist group formed in 2015 and headed by a radical Malian preacher, Amadou Koufa. It is aligned with Al-Qaeda in the region.

- US intelligence support -

In a statement, Macron congratulated the special forces on the operation, in which he also expressed sorrow over the death of the two soldiers "who gave their lives to save those of our citizens".

And Parly 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.

The operation was also made possible by the presence of France's Operation Barkhane, which counts some 4,500 troops deployed in Mali, Burkina Faso, Niger and Chad to help local forces battle jihadist groups.

American special forces and drones are also known to operate in the violence-wracked Sahel region, which France fears could become further destabilised as jihadist groups are pushed out of north Africa, Iraq and Syria.

Burkina Faso has suffered from increasingly frequent and deadly attacks attributed to a number of jihadist groups, including the Ansarul Islam group, the Group to Support Islam and Muslims (GSIM) and Islamic State in the Greater Sahara.

- Relief and sadness -

The French tourists -- Patrick Picque who works in a Paris jewellery shop, and Laurent Lassimouillas a piano teacher, -- went missing with their guide on the last leg of their holiday in usually peaceful Benin.

The Pendjari wildlife reserve, which is famed for its elephants and lions, lies close to the porous border with Burkina Faso.

The badly disfigured body of their guide was found shortly after they disappeared, as well as their abandoned four-wheel Toyota truck.

The two freed men will be flown back to France on Saturday, alongside the South Korean woman, where they will be met on arrival by Macron and other top French officials.

Washington thanked the French forces for freeing the American hostage, with France saying she would likely be "repatriated independently" from the other three.

The two dead French soldiers were named as Cedric de Pierrepont and Alain Bertoncello, decorated naval special forces members born in 1986 and 1991 respectively.

They were part of the prestigious Hubert commando unit of the French naval special forces which was deployed to the Sahel at the end of March.

A total of 24 French soldiers have died in the region since 2013 when France intervened to drive back jihadist groups who had taken control of northern Mali. The last death was on April 2.

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TOYOTA MOTOR