An unusually brutal winter in Mongolia has left much of the country’s grazing land frozen and snow-covered, starving or freezing millions of animals and upending thousands of lives in a country where a third of the population depends on herding and agriculture to make a living.

This year has brought the most snow in 49 years to Mongolia, and the deaths of more than 5.9 million livestock, the worst toll since 2010, international aid groups said this week. While the harshest weather might have passed, about 60 million animals face starvation until new grass sprouts in May, imperiling the future of herding families.

“The worst is yet to come,” Tapan Mishra, the top United Nations official in Mongolia, wrote in a report this week. “The peak of livestock mortality is expected at the end of April.”

The die-off is caused by a weather event known in Mongolia as dzud, where a dry summer is followed by a severe winter that brings deep snow and bitter cold, locking pastures under ice. The deaths can be devastating for families and the country’s economy, 13 percent of which is driven by agriculture, mostly livestock.

This month, Evariste Kouassi-Komlan, UNICEF’s representative in Mongolia, spent nearly three days traveling from the capital, Ulaanbaatar, to a remote western village to deliver medicine. His S.U.V. often got stuck in the snow. Outside each home, called a ger, he found as much as two feet of snow and piles of frozen animal carcasses.

“Some of the herders have lost all of their animals,” he said in an interview. “All of them.”

In eastern Mongolia, Shijirbayar Dorjderem, 48, said that he had lost 800 livestock this year out of the 1,000 he inherited from his parents. That was even after he had purchased thousands of packs of fodder and several tons of wheat, with money borrowed from a bank to feed them over the winter. He said it wasn’t enough to fill their stomachs.

“All I can think about is my bank loan,” he added, afraid the bank might take away his remaining livestock. “I lost almost everything.”

His province, Khentii, was one of the worst-hit by the dzud. Its deputy governor, Oyunbold Lkhagvasuren, said the winter was “merciless.” About 45 percent of the livestock there have died.

Mongolian herders are no strangers to harsh winters. Temperatures can fall to 40 degrees below zero, leaving livestock to freeze to death in a standing position. In 2010, the dzud killed more than 10.3 million livestock, equal to 25 percent of the country’s livestock population, according to the United Nations.

The rising frequency of extreme weather events has made herders’ lives more precarious. Droughts, dust storms, heavy rainfall and flooding have all tripled in the past decade, as temperatures in Mongolia rise twice as fast as the global average. While dzuds used to happen about once every 10 years, this year’s was the fifth in the past decade.

It’s unclear whether the dzud weather pattern is tied to climate change, because no scientific studies looking into possible connections are available yet.

But Mongolia is clearly feeling the effects of climate change in other ways. Average temperatures have increased much faster than the global average (more than 2 degrees Celsius in the past 70 years, according to the United Nations Development Program). Dzuds and droughts are more frequent and more intense.

Mongolia embodies one of the most acute risks of life on a hotter planet. It is familiar with extreme weather, prone to dramatic swings in temperature and precipitation. Climate change — which is caused principally by the burning of fossil fuels, which releases greenhouse gases into the atmosphere — tends to make extremes more extreme and more frequent.

Extreme weather isn’t the only culprit behind the harsh winters. Overgrazing and the depletion of grasslands are the other major factors.

This year’s dzud, which began in November, has left more than 7,000 families in Mongolia lacking adequate food as the livelihoods of thousands of herders, who depend on cattle, goats and horses, were under threat, the International Federation of Red Cross and Red Crescent Societies said last week.

More than 2,000 families have lost over 70 percent of their livestock, the organization added, calling for assistance. Snow has also buried more than 1,000 homes.

The Mongolian government elevated its disaster preparedness level to “high alert” in February, and delivered hay, fodder, food, gas and medical supplies to herders. But aid organizations said more was needed. The United Nations said about $6.3 million was required for the response.

Mr. Kouassi-Komlan, the UNICEF official, said the snow had isolated families, including children who had missed weeks of school. For herders, it might take between five and 10 years to restore their livestock, he added.

“This is a big disaster for these families,” he said.