DIGNE-LES-BAINS, France (AFP) - Nerves are set to jangle in the Tour de France peloton on Wednesday with the hair-raising Col d'Allos descent awaiting them in the Alps.

French talent Thibaut Pinot said the 15.9km descent, in which riders drop more than a vertical kilometre in the space of 15 minutes, is "one of the most dangerous in France".

And the peloton is on red alert after race leader Chris Froome's Sky team-mate Geraint Thomas survived a terrifying moment on Monday's descent into Gap.

The 29-year-old Welshman was baulked by an out-of-control Warren Barguil and sent over a precipice, banging his head on a lamppost along the way.

Thomas was fine to carry on but many riders are likely to be wary on the Col d'Allos descent during Wednesday's 161km 17th stage from Digne-les-Bains to Pra Loup.

"It's a very difficult descent, not so much because it's technical but because the road surface is not very good," said reigning champion Vincenzo Nibali.