The sudden deaths of tens of thousands of endangered antelopes in ex-Soviet Kazakhstan over the past two weeks have left scientists scrambling for answers and conservationists worried about the animal's future.

Over 120,000 rare saiga antelopes -- more than a third of the total global population -- have been wiped out in a devastating blow that the United Nations Environment Programme has called "catastrophic".

UN experts have said the mass deaths are down to "a combination of biological and environmental factors."

Scientists have struggled to put their finger on the exact nature of the disease that has felled entire herds, but say findings point towards an infectious disease caused by various bacteria.

Any infections have likely been exacerbated by recent rains that have made the antelopes -- 90 percent of which live on the steppes of Central Asian Kazakhstan -- less able to cope with diseases.

"Unseasonal wetness may have been something that lowered their immunity to infection but until we do more analysis we will not know anything for sure," Steffen Zuther of the Altyn Dala Conservation Initiative told AFP.

The rate of the deaths has staggered those who have studied the species -- whose ancestors have inhabited the region since the ice age.

"A one hundred percent mortality for the herds affected is extraordinary," said Richard Kock, a professor at the Royal Veterinary College in London who recently returned from Kazakhstan.

"We are dealing with creatures that have fairly low resilience."

The sudden spate of deaths comes as a nasty shock as up until recently the saiga antelopes -- which live for between six and 10 years and are known for their protruding noses -- had been hailed as something of a conservation success.

Until mid-May, when the country's Ministry of Agriculture began reporting the deaths, saiga numbers in Kazakhstan had rallied from an estimated 20,000 in 2003 to the more than 250,000.

In 1993, there were over a million saiga antelopes, mostly concentrated in the steppe land of Kazakhstan, neighbouring Russia and Mongolia.

The susceptibility of the population since then has raised extinction fears and the saiga is listed as critically endangered by the International Union for Conservation of Nature (IUCN).

- Decade to recover -

While herds that have not already been struck down are thought to be safe for the moment, Kazakhstan's Prime Minister Karim Massimov set up a working group including international experts Thursday to establish reasons for the deaths and oversee disinfection of lands in the three regions where the saiga died.

"If there is one positive that has come from this it is that the government has become very open to international channels of cooperation now," Kock from the Royal Veterinary College in London said.

Even then, however, scientists estimate that it will take a decade for the antelope numbers to recover from the recent deaths.

For now though they are hoping that the beasts can avoid even more potent diseases that have raged in nearby areas, such as the morbillivirus epidemic that swept across neighbouring China last year, and other threats.

One of those is the rise in poaching for the animal's horn -- prized in Chinese medicine -- which grew widespread following the collapse of the Soviet Union but has slowed down since.

Kazakhstan extended a ban on hunting the saiga until 2021 four years ago and imposes penalties of up to five years in prison for poachers.