FORT MCMURRAY, Alta. - An explosion and fire at a Syncrude Canada oilsands site in northern Alberta injured one worker as crews fought the flames.

There was a big plume of black smoke at the Mildred Lake upgrader Tuesday afternoon but the smoke was dissipating, said Will Gibson, a Syncrude Canada spokesman.

"Our concern right know is with ensuring the fire is contained and extinguished and people are safe," Gibson said from the site.

Alberta Health Services said emergency medical staff transported one person in serious but stable condition from the Syncrude plant to hospital in nearby Fort McMurray.

The province issued an air quality alert for the Fort McKay area that the fire and smoke could affect health and visibility.

"Take precautions. Stay indoors and keep outdoor activity to a minimum if you have to go outside," said the alert.

"Keep doors and windows closed and use air purifiers if possible. If you suffer from chronic lung conditions, be sure that you have sufficient medication for at least 2 days."

Gibson said the fire broke out shortly before 2 p.m. and the company's fire crews began fighting the flames.

He said access to the site had been restricted and workers were leaving the area.

Syncrude employs about 4,600 people in the region. There was no word on how many people were working at the site when the fire broke out.

RCMP said they responded to the scene of an explosion but Mounties had no information on the cause.

Traffic on Highway 63, the main road to the site - was moving slowly as workers left the area.

"Police are assisting with the evacuation of non-essential services from the Syncrude oil site," RCMP said in a release. "Please do not attempt to enter the Syncrude site or stop off the road on Highway 63."

Melissa Blake, mayor of the Municipality of Wood Buffalo, urged people to stay safe and not to jump to conclusions.

"Anxiety high for those waiting for friends and family to connect," she said on Twitter. "Don't let rumours run the social media mill. Facts will come when they can."

Another fire at the Mildred Lake site in August 2015 cut output from the facility by about 80 per cent. The fire damaged pipes, power and communications lines between two units of the upgrader.

It returned to normal production the following October.