A huge winter storm sweeping across the southern US has killed at least 21 people and left millions without power.
There have been widespread blackouts in Texas, where the energy grid was overwhelmed by a surge in demand.
Millions of people in the state, which rarely experiences such low temperatures, have been struggling to cope with the lack of power and frigid conditions.
The extreme weather is forecast to continue until the weekend.
Deaths attributed to the storm have been recorded in Texas, Louisiana, Kentucky, North Carolina and Missouri.
In pictures: Snow blankets Texas
The National Weather Service (NWS) said more than 150 million Americans were now under winter storm warnings.
And on Tuesday, it reported that more than 73% of the US was covered by snow.
The freezing storm has even reached northern and central parts of Mexico, where millions of people have experienced days of intermittent power cuts.
"I'm in Houston, Texas freezing to death," one Twitter user, Chris Prince, wrote. "No power, no heat, no water. I have four young children. How is this happening right now?"
Another user, Josh Morgerman, wrote that a friend in Texas had resorted to "burning furniture in the fireplace" to stay warm.
'Public health disaster'
The recorded deaths include people who have died in traffic accidents, as well as some who suffered carbon monoxide poisoning from running cars and generators indoors to stay warm.
"This is an absolute public health disaster," one medical official in Houston told the local television station KPRC-TV. "[Carbon monoxide poisoning] certainly happens when it gets cold, but never in these numbers.
One county said it had seen more than 300 suspected carbon monoxide cases during the cold snap. "It's turning into a mini mass casualty event," one Harris County doctor told the Houston Chronicle.
At least four people were killed following a house fire in Houston that officials said may have been sparked by candles. Separately, police said two men found alongside a Houston highway were believed to have died due to the cold.
Meanwhile in North Carolina, a tornado spawned by the same storm left three people dead and 10 injured.
The cold snap has also forced Covid-19 vaccination centres to close for several days and hindered deliveries of doses. Some centres raced to use vaccines that could no longer be refrigerated at the required temperature.
How cold is it in Texas?
The state has seen some of its coldest temperatures in more than 30 years - some areas hit 0F (-18C) on Sunday - and US President Joe Biden earlier approved a state of emergency.
Some four million people in Texas are without power, including 1.4 million in Houston. Around a quarter of homes in Dallas are also experiencing blackouts.
The scale of the power outage has prompted anger from some officials and residents. Governor Greg Abbott said the situation was "unacceptable".
He called for an investigation into the the Electric Reliability Council of Texas (Ercot), an energy co-operative responsible for the vast majority of the state's electricity. He later told local media that the body should be reformed and its leaders should resign.
"This was a total failure," he told ABC News. "They showed that they were not reliable."
In a tweet on Tuesday, Ercot said it was "restoring load as fast as we can in a stable manner".
Homes in the state are not normally insulated for cold weather, meaning that indoor temperatures in homes quickly dropped to freezing after heat systems failed. Frozen pipes also burst, despite attempts by some homeowners to insulate them from the cold using blankets.
Chuck Hairston, who lives in a suburb of Fort Worth, had been without electricity for about 31 hours when he spoke to the BBC on Tuesday.
He said his family had slept beside the fireplace covered in "every blanket and pillow we could find in the house". He tried local hotels, but they were either fully booked or did not have electricity.
Icy roads have also led to a spate of traffic accidents and people have been advised to avoid travel where possible.
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