Two-time defending Tour de France champion Jonas Vingegaard remains in hospital in Spain a day after breaking his collarbone and several ribs in a bad crash with other top riders during the Itzulia Basque Country race.

The Danish rider's Visma-Lease A Bike team said on Friday that further tests revealed Vingegaard had also suffered a collapsed lung and a pulmonary contusion.

The team said cycling's leading star was "stable and had a good night" but remains in a hospital in the northern Spanish city of Vitoria.

The accident comes less than three months before the start of the Tour on June 29 when Vingegaard is scheduled to to again face off against top rival Tadej Pogacar. That highly anticipated rematch is now in doubt.

There was more carnage at the week-long Basque tour on Friday, when Mikel Landa and Soudal Quick-Step teammate Gil Gelders crashed on the fifth stage.

Landa, the runner-up in the race in Spain a year ago, was put into a neck brace and taken away on a stretcher, before being taken to a local hospital where X-rays revealed he'd suffered a fracture to his clavicle.

Romain Gregoire, of Groupama-FDJ, won Friday's stage in a reduced sprint, with Denmark's Mattias Skjelmose of Lidl-Trek still overall leader heading into Saturday's final stage.

Vingegaard was hardly moving as he was put in an ambulance wearing an oxygen mask and neck brace after the crash occurred on Thursday with less than 30km left in the race's fourth stage.

The pile-up also took out cycling stars Primoz Roglic and Remco Evenepoel, as well as Australian ace Jay Vine, who suffered a fractured cervical vertebra and two fractures in his thoracic spine..

Evenepoel broke a collarbone and his right shoulder blade and was set to undergo surgery when he returns to Belgium on Friday, his Soudal Quick-Step team said.

The two-time world champion Evenepoel said in a post on social media that "obviously my plans for the short future will change but I hope and think that my long-term goals will not change."

The 24-year-old is scheduled to make his Tour debut this summer before participating in both the time trial and road race at the Paris Olympics.

The accident happened as riders were making what looked to be a conventional right-hand turn going downhill when one rider's front tyre appeared to slip out and send other cyclists off the road.

There were some large rocks and trees in the area, though it wasn't clear if any of the riders hit them. There was also a concrete drainage ditch on the edge of the curve.

Race director Julian Eraso said that the accident was a surprise since the race organisers considered the curve to be "easy" to handle.

"You never know where an accident can occur," Eraso told Spanish radio Cadena SER. "This year the roads were good, wide, easy roads. That curve to the right was easy ... (and) there was an indication a few metres before to let riders prepare for it."

But Paris-Roubaix race director Thierry Gouvenou named increased speed as one reason for a growing number of crashes at races.

"Stop, stop, stop, let us end the massacre. Let's start to think about the speed problems," Gouvenou told sports paper L'Equipe ahead of Sunday's edition of his race.

"We've made enormous progress in aerodynamics and braking. We're going far too fast.

"I hope that the riders who crashed in the Basque Country will come out OK. But it really is time to set limits."