Investigators have extracted cockpit voice recordings from one of the black boxes of the Airbus plane that smashed into the Alps and expect to have a read-out of their content within days, an official said.

"We just have been able to extract a useable audio data file," Remi Jouty, director of France's BEA air incident investigator told a news conference at its headquarters outside Paris.

But it was too early to draw any conclusions about the causes of the crash, he said.

"There will now be work on the file to understand and interpret the sounds and the voice that can be heard," he said, adding that he expected to have more analysis of the voices in "a matter of days".

The casing of a second black box has been found but not the box itself.

Jouty declined to give details of the recordings. While stressing it was too early to form a clear picture, he said the crash scenario did not appear to be linked to depressurisation and he ruled out a mid-air explosion having taken place.

The last voice message from the Germanwings plane was a routine routing conversation with air traffic control (ATC), Jouty said.

"About a minute later, the trajectory from the ATC show that the plane began a descent and that this descent continued to impact as the last radar position is very close to the impact point. The descent lasted about 10 minutes. The last altitude recorded by radars is very close to the impact point. The altitude at the time was a little over 6,000 feet -- a little over the average altitude of the impact site. That means that the radar followed the plane until impact," he said.