Rescue workers search the mountainside where a Germanwings plane crashed, killing all 150 people on board.

Investigators said the remoteness of the crash site meant it could be days before a clear picture of the tragedy emerged. They said that the crash scenario did not appear to be linked to depressurisation and ruled out a mid-air explosion having taken place.

Most of the victims are German or Spanish. Among them are 16 German schoolchildren returning to Duesseldorf airport after a trip to the Barcelona region.

"On Tuesday last week we sent off 16 happy, young people with two happy, young teachers on a journey," said Ulrich Wessel, headmaster of the Joseph-Koenig-Gymnasium high school.

"It was meant to be a journey full of joy, a school exchange that we've been doing for six years. It ended in tragedy," added Wessel. "Our school will never be the same again."

Investigators said they had extracted cockpit voice recordings from one of the plane’s black boxes and expected to have a read-out of their content within days. They will also need the other black box which records flight data, information that is essential for probing air accidents.

Students and teachers at a small-town German high school broke out in tears when they realised that 16 classmates and two teachers were on board the ill-fated Germanwings airplane.