Native American officials in South Dakota are pleading for immediate assistance as their communities hit by heavy snow run short of essential supplies, reports say.
US media quoted one official as saying that some people are burning clothing to keep themselves warm because they cannot access wood.
More than 30 inches (76 cm) of snow have covered the Pine Ridge Indian Reservation, but strong winds have piled snow drifts 12 feet (3.6 m) high.
Oglala Sioux Tribe President Frank Star Comes Out told BBC News they were fighting a losing battle.
And the governor of South Dakota announced that the National Guard would be activated.
According to reports, machinery is failing as diesel fuel turns to gel in the cold.
As a result, some rescuers have been using horses and sleds, or just walking.
It was unclear how many of the tribe’s 46,000 members were stranded across the reservation’s three million acres, an area roughly the size of Connecticut.