A monster storm barrelling across the United States has killed at least 10 people and prompted warnings to stay off the roads, mass flight cancellations and widespread power cuts as freezing conditions persisted into Monday.

At least 20 states and the US capital, Washington, DC, have declared states of emergency as nearly a million people are without power.

As the storm dumped snow, sleet and freezing rain across a vast area, officials warned that an Arctic air mass trailing the system would drive temperatures to dangerously low levels for days, prolonging disruptions to daily life.

The National Weather Service (NWS) said Americans should expect similar conditions to continue into Monday morning.

New York Mayor Zohran Mamdani said five people were found dead outdoors over the weekend in subzero temperatures. While he did not confirm that the deaths were weather-related, he told reporters there was “no more powerful reminder of the danger of extreme cold”.

In Texas, authorities confirmed three deaths, including a 16-year-old girl killed in a sledding accident.

Two people died in Louisiana from hypothermia, the southern state’s Department of Health said.

The PowerOutage.com tracking site showed more than 840,000 customers without electricity as of Sunday night, mostly in the South, where the storm intensified on Saturday.

In Tennessee, where a band of ice has brought down power lines, more than 300,000 homes and businesses were without electricity while Louisiana, Mississippi and Georgia, where such storms are less common, each reported more than 100,000 outages.

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The loss of power is particularly dangerous as the South is being hit by treacherous cold that the NWS warns could break records.

Authorities have urged residents to remain at home because of the perilous conditions.

Several major airports in New York, Philadelphia and Washington, DC, cancelled nearly all flights for the day.

The brutal storm system is the result of a stretched polar vortex, an Arctic region of cold, low-pressure air that normally forms a relatively compact, circular system but can sometimes elongate into a more oval shape, sending frigid air spilling across North America.

Scientists said the increasing frequency of such disruptions may be linked to climate change although the debate is not settled and natural variability also plays a role.

The NWS warned that heavy ice could cause “long-duration power outages, extensive tree damage, and extremely dangerous or impassable travel conditions”, including in many states unaccustomed to severe winter weather.

Authorities have also warned of life-threatening cold that could persist for a week after the storm, especially in the Northern Plains and Upper Midwest, where temperatures that factor in wind chill are forecast to plunge to -45 degrees Celsius (-50 degrees Fahrenheit), temperatures that can cause frostbite within minutes.