PARIS (REUTERS) - The cockpit voice recorder from the Air Algerie flight that crashed last month in northern Mali, killing all 116 passengers and crew, is unintelligible, investigators said on Thursday.

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 of the plane.

The team has been unable to extract information from one, Remi Jouty, president of France's BEA air accident investigator, told a news conference.

The voice recorder on the 18-year-old aircraft used magnetic audio tape, a system replaced in more recent aircraft by digital technology.

The tape was broken or crumpled in places and had to be repaired but the pilots' conversations still could not be understood.

"There is sound on the tape but it is unintelligible," said Jouty, whose agency has been asked to support Mali's investigation. "The device seemed to be recording but we don't yet know why it did not work, except that this was not a result of the crash itself," he told reporters, adding that first indications were that it was a "simple technical problem".

French officials have said they believe bad weather was the likely cause of the crash of flight AH5017, but have not ruled out other explanations. "We're trying to avoid overly hasty theories," Jouty said.

The pilots had asked for permission to alter their route because of a storm as they flew north after taking off from the capital of Burkina Faso, Ouagadougou.

The jet made a detour to steer around the storm cell, but as it did so it gradually lost some height and speed, according to data from the other black box presented at the BEA's headquarters on Thursday.

After broadly resuming its original course, the aircraft abruptly turned back on itself to the left and entered a corkscrew-shaped descent.

It hit the ground at high speed and the impact was"extremely violent", Jouty said.

The strong concentration of debris in one spot on the ground leads experts to believe that the plane crashed upon impact rather than disintegrating in the air, Jouty said.

A first report will be published in mid-September, said the head of the Malian investigating committee.