The fire followed an explosion at the CNRL Horizon site around 3:30 p.m. MT Thursday. (Submitted)An explosion has rocked the Horizon oilsands site near the Fort McKay First Nation in northern Alberta, injuring four employees.

The explosion occurred around 3:30 p.m. MT Thursday. The owner of the site, Canadian Natural Resources Ltd., said at 7 p.m. MT that the fire was contained to the coker area. A coker uses heat to convert bitumen into crude oil.

Two employees were taken to hospital in Fort McMurray. One person sustained first-degree burns, another suffered second- and third-degree burns.

Two people were treated at the site.

The families of the injured employees have been notified. The company has accounted for all other employees.

A contractor at the site said the blast shot flames and smoke hundreds of metres into the air.

"We heard a massive explosion rock the entire area," said the man, who did not want to be named.

"We thought it was in our unit. We ran out of the unit as quickly as we can. I looked up back up at the coker structure, and it was completely engulfed in flames — a 480-foot [146-metre] tall structure completely engulfed in flames."

A still from video sent to CBC offers another view of the fire. (Submitted)Fort McKay resident Marlene Orr told CBC earlier in the day she could see smoke from her kitchen window.

"I'm looking at a huge wall of black smoke that's filling the sky," Orr said. "We don't know what is being released right now into the air."

CNRL says a mobile air-monitoring unit and officials from Alberta Environment and the Energy Resources Conservation Board are en route. Air-monitoring equipment already on site indicated good quality as of 6 p.m. MT, the company said.

The fire started in an upgrader across from a plant that converts bitumen into crude oil. Production has been suspended at the site, and it is not known when it will resume.

CNRL said the site did not have to be evacuated.