Cockpit voice recordings from an Air Algerie jet that crashed last month in northern Mali are unintelligible, investigators said on Thursday, depriving them of vital clues on what sent it into a sudden plunge that killed all 116 passengers and crew.

The McDonnell Douglas MD-83 aircraft, en route to Algiers, smashed into the ground on July 24 south of the Malian town of Gossi, near the border with Burkina Faso.

Experts in Paris have been examining the two black boxes retrieved from the wreckage. They have been unable to extract information from one, Remi Jouty, president of France's BEA air accident investigator, told a news conference.