Spain's Prime Minister Mariano Rajoy said Thursday he was "deeply shaken" by a French prosecutor's finding that the co-pilot of a Germanwings passenger jet appeared to crash the plane deliberately, killing 150 people.

Rajoy's was the first official public reaction by one of the countries affected by the crash after a prosecutor investigating the disaster said that German co-pilot Andreas Lubitz, 28, appeared to "show a desire to want to destroy" the aircraft.

"Deeply shaken by the latest information given by the investigators. Once again, our heartfelt affection to the families" of the victims, among them 50 Spaniards, Rajoy wrote in a Twitter message.

All 150 people on board the Airbus 320 operated by Germanwings, Lufthansa's low-cost subsidiary, died when it crashed on Tuesday in the French Alps.

At least, 72 Germans and 50 Spaniards were among those on board en route from Barcelona to Duesseldorf in Germany, according to the airline and Spanish authorities.

French prosecutor Brice Robin, said the plane's flight recorders revealed that the captain was out of the cockpit when the jet crashed.

Lubitz "refused to open the door of the cockpit to the pilot and deliberately began the descent of the plane", Robin said.

Rajoy visited the area of the crash on Wednesday along with German Chancellor Angela Merkel and French President Francois Hollande.

Spain declared three days of mourning from Wednesday for the victims.

"We want to join all of them in their pain," Rajoy said of the victims' loved ones in a speech during his visit to France.

"We know that is not easy because what has happened is the worst thing that can befall a human being," he added.

"We want to help them by any means available to us. We want to identify all the victims and repatriate them in the best possible conditions."

A Lufthansa flight took about 60 relatives of the victims from Barcelona to France on Thursday to visit the scene of the disaster.

Escorted by police, the families made no comment to reporters as they boarded buses at a nearby hotel for Barcelona's El Prat airport early Thursday morning.

The spot where the plane crashed is hard to access and closed off while rescuers search the debris, but reception centres have been set up nearby.

A handful of other relatives left Barcelona for France on Wednesday by coach because they did not want to fly, an official said.

Spain has sent a team of police forensics experts to France to help identify the bodies, Spain's junior security minister Francisco Martinez said.