One person is dead and another is in critical condition after an explosion Friday afternoon at Alberta's Nexen Long Lake facility, Wood Buffalo RCMP say.

The person in critical condition was flown by air ambulance to the University of Alberta's burn centre.

The blast occurred at around 4:30 p.m. at the facility located near Fort McMurray, said Staff Sgt. Jeremie Landry.

The Nexen facility is approximately 75 kilometres south of Fort McMurray, outside the hamlet of Anzac.

Brad Grainger, Wood Buffalo's deputy fire chief, said his crew got the call from Nexen at 4:30 p.m. The department sent an ambulance and fire truck to the site and was put on standby.

"Lots of debris and stuff around, and active fire, but it didn't take long for the Nexen crews to get the fire under control and extinguished," Grainger said.

Grainger initially said that the explosion was caused by a coker.

But then later Landry said in the first phase of the RCMP's investigation it appears the blast occurred in the facility's hydrocracker.

On their website Nexen explains that the hydrocracker is used to improve the oil quality and produce synthetic crude by combining the partially upgraded oil with hydrogen to remove sulphur.

RCMP also said the explosion does not appear to be suspicious.

The sign at the Long Lake facility near Fort McMurray. (Rick Bremness CBC)

The Alberta Energy Regulator is on the scene investigating.

"Nexen notified the AER of an explosion on Jan. 15 in the afternoon," said Carrie Rosa, spokeswoman for the regulator.

"They are in the process of shutting down their facility, which is expected to take about 12 hours.

"They have established an emergency response centre at the Long Lake facility, and the AER have dispatched staff to the site."

In a news release, Nexen said its "emergency response plan has been activated, and response personnel, including first responders from the Regional Municipality of Wood Buffalo, are on site.

"We are deeply saddened to confirm one fatality and a second person is at hospital. All other personnel are accounted for."

Nexen said there is no immediate danger to neighbouring communities or personnel still at the site.