Planning is under way to allow evacuated Fort McMurray residents back home, but it's unclear how long it will be before advances in fighting a massive wildfire opens the door to their return, a senior Alberta fire official said Saturday.

However, officials said they will be fighting the overall fire, expected to reach 300,000 hectares in size by day's end Saturday, for months to come even if they make gains that allow the return of more than 80,000 residents to the northern Alberta city.

"It's going to take quite a while for the wildfire fighters to actually get that under control, contain it, put it out," Scott Long, executive provincial operations director for the Alberta Emergency Management Agency, told a Saturday afternoon briefing in Edmonton.

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Efforts to protect the community, including the downtown core and critical infrastructure like the airport, have been "largely successful" he said.

"There is no time limit established yet on a re-entry, but re-entry planning has started," he said.

"Our goal is certainly, as soon as it's stabilized and safe, that we're going to start allowing Albertans to get back to the community," he said. "I am not looking at months."

Officials continue to hope a shift in weather will help, but Chad Morrison, the senior wildfire manager for Alberta, said it would take 100 millimetres of rain to make a difference. He said he was hoping cooler conditions would help hold the fire.

Mr. Morrison traced the fire over the past 24 hours, suggesting it was just over 100,000 hectares on Friday and gained 50,000 hectares by nightfall.

However, he said officials expect the fire to reach between 250,000 to 300,000 hectares by day's end on Saturday.

He also confirmed the fire will "bump up against" the edge of a Suncor oil sands facility today, but the operation has highly trained fire crews of its own and vegetation has been cleared from the sites, limiting fuel for the wildfires.