A state of emergency has been declared in the province of Alberta in Canada after a wildfire forced all 88,000 residents of Fort McMurray to flee.
Officials warn that the fast-moving blaze could destroy much of the city and the next 24 hours will be crucial.
The fire, which broke out on Sunday, has gutted 1,600 structures in the city, including a new school.
The evacuation was the largest in Alberta's history. Nearby oil companies have been forced to cut output.
Several firms operating in Alberta's oil sands region have shut down some pipelines. This was done to help evacuate non-essential personnel, reports say.
So far there have been no reports of deaths or injuries in the wildfire.
First Nation communities 50km (30 miles) south of the city were given mandatory evacuation orders on Wednesday.
The night sky above Fort McMurray is illuminated by a fierce orange glow as fires continue to burn to the north and to the south.
Above the flat landscape of Alberta's famous oil sands is a towering plume of smoke; a giant pyrocumulus cloud so big that it resembles ash from an erupting volcano and has even generated its own lightning.
As we drove north towards the city, hundreds of residents passed us, fleeing to the south, many towing trailers containing as many belongings as they could pack it.
We also passed a fleet of yellow school buses, bound for the town of Anzac and its neighbouring communities, on a mission to rescue the latest people to be threatened by the fast-moving flames.
Fort McMurray itself is sealed off to all but the emergency services but it is reportedly a scene of devastation.
One police officer who lives and patrols in the city told the BBC that in his assessment around half of it had been destroyed.
"You could hear the pop, pop, pop because of the propane tanks," resident Doug Sulliman, a former professional ice hockey player, told Associated Press. "The fire was just consuming these houses. It just destroyed the whole community."
Bernie Schmitte, an official at Alberta's Agriculture and Forestry Ministry, said on Wednesday that the "catastrophic fire" had so far "resisted all suppression methods".
After flying over the burning city, Alberta Premier Rachel Notley said the blaze had moved north and east across Fort McMurray.
If the winds behaved as predicted, she added, the blaze would move into the Thickwood and Timberlea communities, as well as the area around the airport.
"Bad news does not get better with time," said Scott Long, executive director of Alberta's Emergency Management Agency.
"It is a possibility that we may lose a large portion of the town," he added.
Officials said the size of the blaze was now more than 10,000 hectares (39 sq miles) and it was being tackled by 100 firefighters.
Fire services said high winds and hot temperatures were helping the blaze to spread over an even greater area.
Ms Notley praised Albertans for helping neighbours in need.
"We will get through this and we will come out stronger on the other side," she said.
Unseasonably high temperatures and strong winds have combined with dry conditions to leave much of Alberta and neighbouring Saskatchewan under an extreme fire risk warning.
Prime Minister Justin Trudeau said he would send military aircraft to help if they were needed.