BEIJING — Temperatures plunging to -58°F have killed 1.7 million head of livestock in Mongolia, threatening the livelihoods of 21,000 herder families and putting them at risk of food insecurity, warned the Food and Agriculture Organization of the United Nations (F.A.O.).

Urgent external assistance of some $6 million is needed over the next two to three months to help the herders make it through to spring, an F.A.O. rapid-needs assessment on the impact of the disaster has found.

A spell of intense cold followed a very dry and long summer and autumn, which produced insufficient fodder to feed livestock during the winter months. Such extreme weather is known locally as a Dzud. The continuing Dzud has resulted in huge livestock losses, with 1.7 million deaths counted as of Jan. 31. If current conditions persist, the government estimates that losses could reach 3 million to 4 million head of livestock by spring.

One-third of the Mongolian population lead nomadic lives and depend entirely on livestock for a living. Their cattle, sheep, goats, horses and camels are the main household asset and are perishing from cold, exhaustion or starvation. Total economic losses so far are estimated at $62 million. Substantial numbers of wildlife are also dying.

Fourteen of Mongolia’s 21 provinces are considered seriously affected. In eight provinces, 21,000 herder families owning between 100 and 300 head of livestock each have lost more than 50% of their herds, according to the F.A.O. assessment mission.

The F.A.O. mission stressed the urgent need to strengthen household food security for the most vulnerable families to prevent further loss of their assets, and proposed immediate livestock input support for the most vulnerable herders as a top priority. In parallel, fodder, supplementary feed and veterinary care is urgently needed for weak and stressed animals until mid-April, with funding requirements of $6 million.