Three French sports personalities are among 10 people killed in a helicopter crash in Argentina, the French presidential office confirmed.
Eight French nationals and their two Argentine pilots died when two helicopters collided in La Rioja province in the country's north-west.
Yachtswoman Florence Arthaud, Olympic swimmer Camille Muffat and Olympic boxer Alexis Vastine all perished.
Both helicopters were involved in the filming of TV survival show Dropped.
The show, on channel TF1, flies celebrities into rough terrain by helicopter and films their attempts to find food and shelter.
"The sudden death of our fellow French nationals is a cause of immense sadness," said a statement from French President Francois Hollande.
The cause of the crash is as yet unclear and investigations are under way.
The three sports stars killed
Camille Muffat, 25, won three medals, including gold in the 400m freestyle in the 2012 London Olympics. She retired from competitive swimming in 2014.
Alexis Vastine, 28, won bronze at the 2008 Beijing Olympics in the light-welterweight category. He was narrowly beaten in the quarter-finals of the 2012 Games.
Florence Arthaud, 57, was a hugely accomplished sailor, winning the 1990 Route du Rhum, the prestigious solo Atlantic race. A serious car accident put her in a coma when she was 17.
The helicopters collided during filming near Villa Castelli, about 1,170km (720 miles) from Buenos Aires, according to local officials.
Five of those killed were said to have worked for Adventure Line Productions (ALP), the company making the programme.
They were named as Laurent Sbasnik, Lucie Mei-Dalby, Volodia Guinard, Brice Guilbert and Edouard Gilles.
The Argentine pilots were named as Juan Carlos Castillo and Roberto Abate.
Other contestants were standing on the ground blindfolded a few hundred metres away when the accident happened, French media said.
French prosecutors have opened an inquiry into the deaths - standard procedure when nationals die overseas.
Tributes have poured in to the dead stars from France and beyond.
"We are shocked by this sad news," said Thomas Bach, President of the International Olympic Committee.
"Our thoughts are with the families and friends of these great athletes who have left us."
"The world of sport and the Olympic family have lost three of their key members," he said, calling them champions and role models.
"Florence's view was that the sea is out there and it's there for the taking," the British round-the-world yachtswoman Dame Ellen MacArthur told the BBC.
"You know, I'm sure she's inspired many, many people."
"I'm distraught. Immense grief," said Christian Estrosi, the mayor of Nice, Muffat's hometown.
"I saw her grow up a little as if she were my daughter. I'm thinking of her parents. Camille, we miss you."
France in mourning - by Lucy Williamson, BBC News, Paris
France is waking up to the shock of its loss, with the death of the three sports stars and five other French nationals splashed across every paper, every news bulletin.
Prime Minister Manuel Valls tweeted his "immense sadness" saying "all France is grieving this morning".
Meanwhile Culture Minister Fleur Pellerin told the BFM-TV news channel that the three athletes were "among the most outstanding ambassadors for our country".
But amid the horror there are also the first signs of anger. Well-known musician Benjamin Bioley has criticised what he called a "horrible TV reality show that serves up cardboard stars and then steals three wonderful athletes".
As the country begins to absorb the news, the questions over this incident and the circumstances that led to it will only grow.
Filming had begun late last month at Ushuaia in the deep south of Argentina and had since moved to the mountainous western province of Rioja.
One cast member, figure skater Philippe Candeloro, was said to be "extremely shocked but unharmed".
Officials have said the weather conditions in the mountainous area where the helicopters went down were good.
"An explosion occurred and it's believed that they must have collided,'' said La Rioja Secretary of Security Cesar Angulo.
"Aeronautical experts will have to determine that," he added.
"We learn with great sadness about the accident during the filming of the Dropped show," said a statement issued by TF1.
"All TF1 teams come together in this terrible time with the pain of the families and relatives of the victims."
TF1 says the show will be postponed and the rest of the crew is returning home.
Reality TV meets tragedy
- 2013: On Koh Lanta show produced by ALP for TF1, Gerald Babin, 25, dies of a heart attack. Days later the doctor who treated him, Thierry Costa, 38, kills himself complaining of bad media coverage
- 2010: Phil Harris, a fisherman, suffers a huge stroke while unloading crab during filming for Discovery Channel's Deadliest Catch reality series and dies days later. The episode featuring his death is later broadcast, followed by a special tribute episode
- 2009: a contestant on the Bulgarian version of Survivor, Noncho Vodenicharov, 53, suffers a heart attack in the Philippines and dies
- 2009: Saad Khan, a 32-year-old father of four, drowns while taking part in a swimming challenge on a Pakistani reality TV show
- 2006: Australian Steve Irwin, international wildlife expert and star, dies off Queensland coast after being pierced with a stingray barb while shooting a film called Ocean's Deadliest