Nearly seven million people in the US state of Texas have been told to boil tap water before consuming it after a deadly winter storm caused power blackouts at treatment facilities.
The huge storm sweeping across the southern US has killed at least 24 people and left millions without power.
Texas has seen widespread outages.
Freezing temperatures have also caused water pipes to burst, despite attempts by some homeowners to insulate them from the cold using blankets.
The state's energy grid has been overwhelmed by a surge in demand as people try to keep warm in some of the coldest temperatures there in more than 30 years - hitting 0F (-18C) earlier this week.
Over 13 million people - close to half of the state's population - have faced some disruption of water services.
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US President Joe Biden earlier approved a state of emergency for Texas, which has been blanketed by snow and ice since the storm swept in at the weekend.
What's happening in Texas?
A so-called "boil water notice" has been issued for the city of Houston and surrounding areas, with the US Centers for Disease Control and Prevention (CDC) advising that all water planned for consumption - even if filtered - must be boiled as it may be contaminated.
Only boiled or bottled water should be used, including for brushing teeth, the CDC said.
The mayor of Houston, Sylvester Turner, urged those without power - and unable to boil tap water - to try to access bottled water wherever possible.
"Power will not be restored fully, I would say, probably for another couple of days," he told reporters at a press conference.
Houston police and disaster response teams will be delivering bottled water to those with disabilities who do not have access to transportation, the mayor said on Thursday.
Mr Turner also urged people who had been leaving taps running to prevent pipes from freezing not to do so, in an effort to help conserve supplies.
Homes in the state are not normally insulated for cold weather, meaning that indoor temperatures quickly plummeted after heating systems failed.
Mr Turner said that businesses and churches were being used as "warming centres" to help shelter some of the most vulnerable, while also providing bottled water to residents.
"We are still in the midst of this winter storm," he warned, urging people to avoid the roads or travelling in the mornings and evenings, describing them as the "most dangerous times".
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As of Thursday, 900 public water systems have been affected by the winter storm, a spokesman from the state's Commission on Environmental Quality told the Texas Tribune newspaper.
Governor Greg Abbott said that 1.2 million Texans had so far had their power restored, with more coming "on board". As of Thursday afternoon, over 382,000 people were still without power, according to the PowerOutage.us tracker.
The governor signed an executive order preventing any Texas natural gas from being exported outside of the state until 21 February.
Mr Abbott also told reporters that every source of power in Texas had been "compromised" - seemingly reversing earlier comments blaming renewable energy for the state's blackouts.
Chuck Hairston, who lives in a suburb of Fort Worth, had been without electricity for about 31 hours when he spoke to the BBC earlier this week.
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.
Meanwhile, Covid-19 vaccination appointments, which had been postponed due to centre closures and hindered deliveries of doses, were scheduled to resume next week, Mr Turner said.
By Thursday afternoon, the National Weather Service (NWS) said the system was gradually loosening "its icy grip over the Heartland", but warned that "abnormally cold temperatures will hang around through the weekend".
On Wednesday, more than 71% of the US was covered by snow.
How deadly is the storm?
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.
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