OTTAWA — Walls of flame driven by strong, shifting winds raged out of control on Wednesday in and around the evacuated city of Fort McMurray, Alberta, where firefighters were helpless to stop the destruction and where about .

“To date, the fire has resisted all suppression efforts,” Bernie Schmidt, an Alberta forestry official, told reporters in a conference call on Wednesday. “This is a very complex fire, with multiple fronts and explosive conditions.”

Rachel Notley, the premier of the province, said that at least 1,600 buildings had been destroyed. No deaths or serious injuries were reported, but the danger was far from over.

“This is a really dirty fire,” Darby Allen, the regional fire chief for the area, said on the conference call. “There are certainly areas within the city which have not been burned, but this fire will look for them, and it will take them.”

The entire population of Fort McMurray, the main center for Canada’s region, was on Tuesday evening once the fire, which began in woodlands outside the city, had overwhelmed firefighters’ efforts to hold it at bay. Cars and trucks jammed the only route out of the city, Highway 63, which runs north to the -sands work camps and south to Edmonton, the nearest sizable city, 270 miles away.

As the road became gridlocked, it took motorists five hours or longer to cover 12 miles to evacuation centers at the work camps. And southbound travel was impossible for several hours on Tuesday when flames blocked the highway. Some motorists took to Highway 63’s grassy median and even drove the wrong way on the opposite side in hopes of escaping the city faster.

Jasmin Herold, a German filmmaker who has lived in Fort McMurray for the last two years, had a harrowing time getting out. Like many in the city, she said she thought the worst of the fire had passed when Tuesday dawned bright and sunny. But when the smoke returned in midafternoon, she gathered up her dog, her notes, laptop and hard drives with documentary footage and jumped into a small Toyota sedan with her boyfriend, Michael Beamish.

Getting from her neighborhood to Highway 63, normally a two-minute drive, took an hour and a half, Ms. Herold said in an interview, and all the while “through the back, I saw the flames reaching for our area.” Highway 63 was jam-packed, so they tried local roads to get to the south end of town, and found themselves driving through a tunnel of fire.

“It was hot in the car, very hot — I thought maybe this was it for us,” she said. With little choice, they pressed on, she said, and finally “went up a hill and into sunshine again.”

The southbound highway was littered with abandoned cars, trucks and buses. The evacuation had drained Fort McMurray of gasoline, and many motorists ran out of fuel in the clog of traffic fleeing the city. The jams had cleared by Wednesday afternoon, and gasoline tanker trucks began patrolling the highway to refuel stranded vehicles.

Hundreds of firefighters were in Fort McMurray on Wednesday, with more on the way, but many of them were standing by, waiting for a chance to go into action. Helicopters and airplanes dropped water and chemical suppressants, but the speed of the fire’s progress and frequent shifts in its direction have so far made it too dangerous to tackle it from the ground, said Laura Stewart, a spokeswoman for the provincial government.

As the fire spread into the city, reporters and witnesses said, the sound of exploding propane tanks filled the air. The downtown business district of Fort McMurray was still largely undamaged on Wednesday, but several businesses on its fringes were destroyed, including a gas station and a hotel, and there were signs that the fire was once again headed its way.

Prime Minister Justin Trudeau said on Wednesday that he had pledged the national government’s “total support” for the firefighting efforts, aid for evacuees and other needs during a telephone call with Ms. Notley, the premier, who flew to Fort McMurray on Wednesday. Mr. Trudeau said offers of support from local governments across Canada had been pouring in as well.

“Canada is a country where we look out for our neighbors,” Mr. Trudeau said in Ottawa, where he was meeting with lawmakers of his party.

The defense minister, Harjit Sajjan, told reporters on a conference call that the military was prepared to offer whatever assistance was requested. Alberta is home to several large military bases.

Most of Fort McMurray’s operations are north of the city and well outside the likely path of the fire. Ian D. Gates, chairman of the department of petroleum engineering at the University of Calgary, said that it was unlikely that the oil sands deposits underground would be affected, but that the bitumen in oil sands already at the surface, exposed by mining or erosion, might burn if a sufficiently hot wildfire were to reach them.

Thousands of oil sands workers live in other parts of Canada and commute by air to the region for a few weeks’ work at a time, sleeping in company-owned or leased work camps between shifts. Several oil companies shut down or curtailed operations and flew employees out to make room in their camps for evacuees.

The city’s airport remained open on Wednesday, but only for a limited number of departing civilian flights. Military aircraft arrived carrying emergency personnel and supplies. WestJet, a major Canadian airline, brought in aircraft on Tuesday to evacuate more than 100 patients from Fort McMurray’s hospital.

Ms. Stewart, the provincial spokeswoman, said the immediate cause of the fire, which started over the weekend, was still unknown. But she added that the winter and spring had been unusually dry and warm, parching the forests that surround the city. The weather this week has been unseasonably hot, with temperatures on Tuesday reaching about 90 degrees Fahrenheit — rare in Fort McMurray even at the peak of summer — with relative humidity of just 13 percent.

Fire and emergency services officials said that shifting winds and continued high temperatures could increase the size of the fire, which covered about 25,000 acres on Wednesday morning. Satellite thermal images showed that the fire jumped a river on Wednesday and was curling back toward downtown Fort McMurray, on one hand, and toward the city’s airport and an industrial park on the other. Firefighters spent Tuesday night clearing fire breaks near the industrial park and airport, but the intensity of the fire made it unclear whether those efforts would have any success.

Ms. Notley, the provincial premier, said that officials were holding off on flooding the area with assistance until the shape of the fire becomes clearer and the weather turns cooler, which may help make the fire less destructive.

“Your first thought is to say: ‘Get more people, get more stuff, go, go, go,’ ” Ms. Notley said, but that approach could cause added danger and hamper firefighting work by clogging the city’s small airport. Instead, she said, the province has been concentrating on sending specially trained firefighters and specialized equipment in from other parts of the province.

David Martell, a professor in the Fire Management Systems Laboratory of the University of Toronto, said that large forest fires are effectively impossible to fight in hot, dry conditions, because they can shoot out flames more than a mile ahead of their fronts. All that crews can do right now, he said, is prepare plans and resources to “really go in and hammer it” when the weather changes.