At least 20 people have died after a winter storm in the US left millions without power in record-breaking cold weather.
The worst US power outages were in Texas, affecting more than two million homes and businesses.
Texas officials requested 60 generators from the US' Federal Emergency Management Agency and planned to prioritise hospitals and nursing homes.
The state opened 35 shelters to more than 1,000 occupants, the agency said.
More than 500 people sought comfort at one Houston shelter. Mayor Sylvester Turner said other warming centres were closed because they lost power.
Always Ready, Always There! https://t.co/QPNNwL7cVG— National Guard (@USNationalGuard) February 16, 2021
Wind-chill warnings extended from Canada into Mexico with the weather likely to delay COVID-19 vaccine shipments and deliveries.
Texas officials said more than 400,000 vaccine doses due now will not arrive until at least Wednesday because of the storm.
The National Weather Service was “very surprised how rapidly this storm intensified ... and at the time of night when most people are at home and in bed, it creates a very dangerous situation,” Emergency Services Director Ed Conrow said.
Utility companies had to implement rolling blackouts to ease the burden on power grids straining to meet extreme demand for heat and electricity.
Several cities had record lows: In Omaha, the temperature bottomed out at minus 30 degrees Celsius, the coldest in 25 years.
One Minnesota weather station registered minus 39 degrees Celsius. Sioux Falls, South Dakota, dropped to minus 26 degrees Celsius.
At midday, more than 2,700 US flights had been canceled, 1,200 of them at Texas airports.
Authorities pleaded with residents to stay home on Tuesday and forecasters said conditions might not improve until temperatures rise above freezing Wednesday afternoon.