An explosion and fire shut down Syncrude's Mildred Lake oilsands upgrader north of Fort McMurray on Tuesday afternoon.
Alberta Health Services said one person was transported from the plant to the city's Northern Lights Regional Health Centre in serious but stable condition.
Wood Buffalo RCMP and other emergency services are on the scene, and Alberta Energy Regulator investigators have also been dispatched to the site.
Police said in a news release they were unable to confirm the cause of the explosion.
Syncrude spokesperson Will Gibson said the fire started just before 2 p.m. MT. He said he did not have any information about injuries.
"There's no smoke anymore," Gibson said just before 4 p.m., though he could not confirm that the fire had been extinguished.
"From my vantage point on site, everything appears normal," he said. "There's nothing amiss. But I'm not in a position or vantage point to say the fire is contained or extinguished."
RCMP said they have shut down one southbound lane of traffic on Highway 63, the main road that connects the oilsands site with Fort McMurray.
Police said they were assisting with the evacuation of non-essential services from the site.
The Regional Municipality of Wood Buffalo offered its emergency services to support the company's own firefighters in battling the blaze.
Melissa Blake, mayor of the regional municipality, told CBC News that Tuesday's fire at one of the Alberta's vital oilsands facilities likely has many residents thinking back to last May, when a massive wildfire forced more than 90,000 people from their homes.
"On the outside, looking at social media, the videos that are being posted, people immediately think (about) who they know that's there," Blake said. "And I think the anxiety comes from just not knowing what all the answers are at this point."
Emergency responders will have much the same concerns as they did back in May 2016, Blake said, to make sure "the right personnel are in the right place at the right time."
"The better information that we're able to get out to the public, the more meaningful the reactions can be, but at this point we don't have details," she said. "We're going to get this sorted out, we'll get through it together as we did with the fire."
A fire at the company's Mildred Lake Base plant in August 2015 was quickly extinguished and the company said at the time there were no reports of injuries.