The chief executive of Lufthansa says he is "stunned" by a French prosecutor's conclusion that the co-pilot of a Germanwings plane intentionally caused Tuesday's crash which killed 150 people.

Germanwings is Lufthansa's budget-price subsidiary.

Chief executive Carsten Spohr told a news conference in Cologne, Germany that "we choose our staff very, very carefully." He says the airline had no indication of why the co-pilot would have crashed the plane. He said pilots undergo yearly medical examination but that doesn't include psychological tests.