The co-pilot of the Germanwings plane that crashed into the French Alps on Tuesday appeared to want to "destroy the plane", French officials said.

Marseille prosecutor Brice Robin, citing information from the "black box" voice recorder, said the co-pilot was alone in the cockpit.

He intentionally started a descent while the pilot was locked out.

Robin said there was "absolute silence in the cockpit" as the pilot fought to re-enter it.

Air traffic controllers made repeated attempts to contact the aircraft, but to no avail, he said.

Passengers could be heard screaming just before the crash, he added.

The co-pilot, now named as Andreas Lubitz, 28, was alive until the final impact, the prosecutor said.

The Airbus 320 from Barcelona to Duesseldorf hit a mountain, killing all 144 passengers and six crew, after an eight-minute descent.

Lufthansa Chief Executive Carsten Spohr and Germanwings Managing Director Thomas Winkelmann (L) arrive for a news conference in Cologne Bonn airport. Photo:Reuters

"We hear the pilot ask the co-pilot to take control of the plane and we hear at the same time the sound of a seat moving backwards and the sound of a door closing," Robin told reporters.

He said the pilot had probably gone to the toilet.

"At that moment, the co-pilot is controlling the plane by himself. While he is alone, the co-pilot presses the buttons of the flight monitoring system to put into action the descent of the aeroplane.

"This action on the altitude controls can only be deliberate."

He added: "The most plausible interpretation is that the co-pilot through a voluntary act had refused to open the cabin door to let the captain in. He pushed the button to trigger the aircraft to lose altitude. He operated this button for a reason we don't know yet, but it appears that the reason was to destroy this plane."

He said the co-pilot was "not known by us" to have any links to extremism or terrorism.

But he said German authorities were expected to give further information on his background and private life later.

Passengers were not aware of the impending crash "until very last moment" when screams could be heard, Robin said, adding that they died instantly.

Meanwhile, relatives and friends of the victims are due to visit the area of the crash.

Lufthansa, which owns Germanwings, arranged two special flights for families and friends on Thursday - one from Barcelona and one from Duesseldorf - to Marseille, and both groups will travel on by road. Separately, some relatives who did not want to fly are travelling by bus from Barcelona.

The second "black box" - that records flight data - has still not been found.