A disease has killed 2,500 critically endangered Saiga antelope in Western Mongolia since the beginning of the year, scientists say.
The Wildlife Conservation Society (WCS) scientists told the BBC that the deaths were caused by a virus known as PPR.
The disease, which originated in livestock, could now threaten Mongolia's entire Saiga population, the researchers said.
There is concern for the impact on the wider grassland ecosystem, particularly on the populations of animals the snow leopard preys on.
"Many other species share this same range," WCS veterinary scientist Enkhtuvshin Shiilegdamba said, "including ibex and big-horned sheep."
The Saiga carcasses are burned to prevent the spread of the disease and domestic livestock in the affected area have been vaccinated.
But Amanda Fine, a vet with the WCS, said "further immunization" was needed in both Saiga range areas and in the habitats of other affected species.
A distinctive Saiga antelope subspecies inhabits the steppes and semidesert regions of Kazakhstan and Kalmykia in Russia, although in winter some animals reach Uzbekistan and northern Turkmenistan, according to the International Union for Conservation of Nature and Natural Resources (IUCN).