Fire raged unchecked through the Canadian city of Fort McMurray overnight as authorities raced to complete the evacuation of its population of 80,000, fearful that hot, dry winds forecast for Wednesday would further fan the flames.

About 44,000 people were estimated to have fled the city by late on Tuesday on traffic-chocked roads, and the province of Alberta requested military help to bring the blaze under control and airlift others from fire- and smoke-filled streets.

"I'm afraid that huge parts of my home town... may burn tonight and will continue to burn," Brian Jean, leader of Alberta's official opposition party, told CBC Radio, saying his own home was in the immediate path of the flames.

The fire in the heart of Canada's oil sands region broke out southwest of the city on Sunday, shifting aggressively with the wind to breach city limits on Tuesday, when its size was estimated at 26.5 square kilometres (6,540 acres).

It destroyed one residential neighbourhood in the southeast, and others were severely damaged or under threat, Chief Darby Allen of Fort McMurray's fire department said.

The blaze also temporarily closed off the main southern exit from the city, Highway 93, prompting many residents to flee north towards the oil sands camps.

Alberta Premier Rachel Notley described the evacuation as the biggest in the province's history.

Officials said army and air force assistance would take two days to arrive, and they expected to face another day of battling the flames on Wednesday, when relatively low humidity, hot temperatures and high winds were in the forecast.

"We may, to some extent, still be in fire extinguishment mode," Fire Chief Allen told a news conference on Tuesday night. "I think, based on the conditions ... we're going to face a day that's fairly similar to today."

Nine air tankers, more than a dozen helicopters and about 150 firefighters were deployed to combat the flames, officials said.

Suncor Energy, whose oil sands operations are closest to the city, said its main plant 25 km (16 miles) to the north was safe, but it was reducing crude production in the region to allow employees and families to get to safety.

ONE BIRTH, NO CASUALTIES

Officials said around 8,000 evacuees had reached a reception centre outside Fort McMurray, while Premier Notley told reporters that authorities were working to set up more emergency accommodation.

No casualties had been reported, the Regional Municipality of Wood Buffalo said early on Wednesday on Twitter, and its mayor Melissa Blake said at least one baby was born at a lodging for energy workers being used as an evacuation site.

"My thoughts are with people affected by the fire in Fort McMurray tonight," Prime Minister Justin Trudeau tweeted on Tuesday. "Stay safe and remember to follow evacuation orders."

Suncor said evacuees were welcome at its Firebag oil sands facility, while Canadian Natural Resources Ltd said it was working to ensure any affected CNRL workers and their families could use its camps.

Shell Canada said it would open its oil sands camp to evacuees and was looking to use its airstrip to fly out non-essential staff and accommodate displaced residents.

Representatives of Syncrude, CNOOC subsidiary Nexen Energy and pipeline company Enbridge all said their operations were unaffected.

A number of flights from Fort McMurray airport were cancelled on Tuesday.

The fire is the second major one in the oil sands region in less than a year. Last May, wildfires led to the evacuation of hundreds of workers, and a 9 percent cut in Alberta's oil sands output as Cenovus Energy and Canadian Natural Resources Ltd called shutdowns at some of their projects.

Alberta could be in for long and expensive wildfire season this year as it is much drier than normal after a mild winter with lower-than-average snowfall and a warm spring.

Fort McMurray is located about 430 km northeast of Alberta's capital, Edmonton. Most oil sands facilities are to the north and east of the city. – Reuters