Canada's leading oil company Suncor will produce less crude than planned after massive forest fires in the city of Fort McMurray, Alberta delayed operations last month, the firm said Monday.

The wildfires forced some 100,000 residents of Fort McMurray and surrounding areas of the oil sands hub to evacuate. Residents started returning last week after the fires moved away from town, tracking eastward.

"Due to the cumulative impact of the fires on refinery feedstock and a short unplanned outage at one unit of Suncor's Edmonton refinery, gasoline and diesel production has been reduced," the company said in a statement. 

At Suncor affiliate Syncrude, which also had to stop work last month, production should be back to normal by mid-July, Suncor said.

The company now forecasts total production of 585,000 to 620,000 barrels a day for 2016 -- still higher than last year's.

Before the fires, it was targeting 620,000 to 665,000 barrels per day.

The first convoys of weary, anxious residents returned to wildfire-ravaged Fort McMurray on Wednesday, a month after they were forced to flee.

The downtown was unscathed and most city services have been restored, except for potable water. Most stores have restocked.

However, the authorities have fenced off three hard-hit neighborhoods by a 30-kilometer (19-mile) enclosure. The few undamaged homes were deemed unsafe for habitation this week after tests of air, soil and ash revealed chemical and heavy metal contamination.

The fire continues to burn away from populated areas, growing only minimally in the last few days to more than 580,000 hectares (1.4 million acres).

Some 1,700 firefighters, including teams from South Africa and the United States, continue to battle the blaze, assisted by water bombers and heavy equipment.