The pace of Fort McMurray residents' return to the wildfire-ravaged Canadian oil city accelerated Thursday, but continued to fall short of expectations.

Authorities were prepared to welcome back as many as 40,000, more than double the previous day's count, but early indications suggested that the target would not be met.

"Based on traffic flow… we expect approximately 20,000 people will voluntarily come back to their community today," Scott Long, head of the Alberta Emergency Management Agency, told a press conference.

The repatriation kicked off the day before, with 8,000 people returning to their homes on Wednesday, drawing back curtains and ditching fridges full of spoiled food after a month away.

"I'm pleased to report that the voluntary phased re-entry of residents went very smoothly yesterday," Long said.

He added however that only half the number of people eligible to return in the first wave did so.

The fire, which forced the evacuation on May 3 of nearly 100,000 people from Fort McMurray and surrounding villages, remains out of control.

But it has moved away from populated areas as it tracks eastward, while its size has stabilized at 580,000 hectares (1.4 million acres).