More than 500,000 households and businesses in the US are without power and more than 9,600 flights are expected to be cancelled ahead of a monster winter storm that threatens to paralyse eastern states with heavy snowfall.

Forecasters said snow, sleet, freezing rain and dangerously frigid temperatures would sweep the eastern two-thirds of the nation on Sunday and into next week.

Calling the storms “historic”, Donald Trump on Saturday approved federal emergency disaster declarations in South Carolina, Virginia, Tennessee, Georgia, North Carolina, Maryland, Arkansas, Kentucky, Louisiana, Mississippi, Indiana, and West Virginia.

“We will continue to monitor, and stay in touch with all States in the path of this storm. Stay Safe, and Stay Warm,” Trump wrote in a post on Truth Social.

Seventeen states and the District of Columbia had declared weather emergencies, the Department of Homeland Security (DHS) said. The DHS secretary, Kristi Noem, at a news conference on Saturday, warned people to take precautions.

“It’s going to be very, very cold,” Noem said. “So we’d encourage everybody to stock up on fuel, stock up on food, and we will get through this together.” She added: “We have utility crews that are working to restore that as quick as possible.”

The number of outages continued to rise. As of 6.30am EST (1130 GMT) on Sunday, more than 400,000 US customers were without electricity, according to PowerOutage.us, with 100,000 each in Mississippi and Texas. Other states affected included Louisiana, Tennessee and New Mexico.

The Department of Energy on Saturday issued an emergency order authorising the Electric Reliability Council of Texas to deploy backup generation resources at data centres and other major facilities, aiming to limit blackouts in the state.

On Sunday, the department issued an emergency order to authorise the grid operator, PJM Interconnection, to run “specified resources” in the mid-Atlantic region, regardless of limits due to state laws or environmental permits.

The National Weather Service warned of an unusually expansive and long-duration winter storm that would bring widespread, heavy ice accumulation in the south-east, where “crippling to locally catastrophic impacts” could be expected.

Forecasters predicted record low temperatures and dangerously cold wind chills descending further into the Great Plains region by Monday.

More than 9,600 US flights scheduled for Sunday had been cancelled, according to flight tracking website FlightAware, with more than 4,000 flights cancelled on Saturday.

Major US airlines warned passengers to stay alert for abrupt flight changes and cancellations. Delta Air Lines adjusted its schedule on Saturday, with additional cancellations in the morning for Atlanta and along the east coast, including in Boston and New York City. It would relocate experts from cold-weather hubs to support de-icing and baggage teams at several southern airports, the airline said.

JetBlue said that as of Saturday morning it had cancelled about 1,000 flights through Monday. United Airlines said it had cancelled some flights in places with the worst expected weather.

US electric grid operators on Saturday stepped up precautions to avoid rotating blackouts. Dominion Energy, whose Virginia operations include the largest collection of datacentres in the world, said if its ice forecast held, the winter event could be among the largest to affect the company.