The German, French and Spanish leaders have arrived together in the French Alps to visit the scene where a Germanwings plane crashed on Tuesday, killing all 150 on board.
Angela Merkel, Francois Hollande and Mariano Rajoy were briefed on the rescue and thanked the workers.
One of the "black box" recorders has been recovered at the site near Digne.
The Airbus A320 - flight 4U 9525 - from Barcelona to Duesseldorf crashed after an eight-minute rapid descent.
Germanwings chief Thomas Winkelmann said 72 of the 144 passengers were German citizens. The victims included 16 pupils returning from an exchange trip. Spain's government now says 51 of the dead were Spanish.
UK Foreign Secretary Philip Hammond confirmed that three Britons were on board. Other victims were from Australia, Argentina, Iran, Venezuela, the US, the Netherlands, Colombia, Mexico, Japan, Denmark and Israel.
However, there may be some duplication because of dual citizenship.
The plane's cockpit voice recorder - recovered by a helicopter team on Tuesday - was damaged but could still provide information, French Interior Minister Bernard Cazeneuve said.
Investigators are still searching for the second "black box" - the flight data recorder.
Analysis by Richard Westcott, BBC Transport Correspondent
The "black box" cockpit voice recorder is pretty battered but the devices are bomb-proof, literally, so investigators should still be able to listen to what happened on board.
The large, orange cylindrical part in the middle holds the memory boards that record the pilots' voices, and that looks intact to me.
The critical question now is, were the pilots speaking during those lost eight minutes when the aircraft dropped from 38,000ft to the ground?
If they're quiet, investigators will know they were unconscious, and that suggests a major decompression took place on board. That's when a hole in the fuselage lets all the air out. It's not necessarily fatal, if the pilots can get their oxygen masks on in time, something they train for, but if they were knocked out, it would explain the lack of a mayday call or any obvious attempt to steer the stricken aircraft to the nearest runway.
A day of mourning is being held at the Joseph-Koenig-Gymnasium school in Haltern-am-See, north-west Germany, where the 16 German pupils were from.
Pupils at the Giola secondary school in the village of Llinars del Valles, north-east Spain, held a private 15-minute ceremony of remembrance for their foreign exchange student friends.
After arriving near the crash site, French President Hollande, German Chancellor Merkel and Spanish Prime Minister Rajoy thanked the rescue workers for their efforts in difficult circumstances.
At the scene: James Reynolds, BBC News, Seyne les Alpes
French gendarmes here have taken over a large part of this valley on the outskirts of town. One van with sirens escorted in a white trailer bearing the words "Mobile Criminal Investigation Laboratory".
Emergency workers in fluorescent jackets say that they want to work as quickly as they can.
"We cannot leave the victims a week in the mountains," said Roy Xavier from Civil Security.
But victims' families will have a long wait for the bodies to be returned to them. The Prosecutor of Marseille told reporters that it may be several weeks before all the victims are identified.
Mr Rajoy has declared three days of national mourning in Spain.
Bereaved relatives have also begun to arrive at the scene of the crash. The mayor of Seyne-les-Alpes, the town nearest the site, said local families were offering to host them.
Footage shot from a helicopter on Tuesday showed plane parts scattered on the rocky mountainside.
"The site is a picture of horror," German Foreign Minister Frank-Walter Steinmeier said after being flown over the ravine.
Germanwings, a low-cost airline owned by Germany's main carrier Lufthansa, said some crew members were unfit for service on Wednesday "due to emotional distress".
It said one flight was being cancelled but remaining flights would be according to schedule.
Lufthansa and Germanwings staff held a minute's silence on Wednesday morning.
Meanwhile, the first Germanwings flight on the same route as the crashed plane took off from Barcelona on Wednesday morning but the flight number had been changed to 4U 9441.
No distress signal
Flight 4U 9525 began descending one minute after it reached its cruising height and continued to lose altitude for eight minutes, Germanwings managing director Thomas Winkelmann told reporters.
He said the aircraft lost contact with French air traffic controllers at 10:53 (09:53 GMT) at an altitude of about 6,000ft.
The plane, a single-aisle passenger jet popular for short- and medium-haul flights, did not send out a distress signal, officials said.
The White House has said there is no evidence so far of a terror attack. A Lufthansa official said they were assuming for the time being that the crash had been caused by an accident.