Image caption The Alberta oil sands are the largest petroleum reserve outside of the Middle East

An explosion and fire at the Horizon Oil Sands facility in Canada's Alberta province has injured four workers and forced the site to suspend operations.

The blast at the plant, owned by Canadian Natural Resources Ltd, ignited the upgrader, part of the plant that converts bitumen into synthetic crude.

The company said the fire had been isolated and brought under control.

Alberta's oil sands are the largest petroleum reserve outside the Middle East.

Plumes of smoke

Oil production has been halted at Horizon Oil Sands and it is unclear when it will resume, Canadian Natural Resources said in a statement.

The blast, which occurred at 1530 local time (2130GMT) in northern Alberta on Thursday, sent flames and smoke hundreds of metres into the air.

Four workers were injured in the incident, with three sent to hospital, Canadian Natural Resources said.

One of the workers was reportedly treated for second and third-degree burns, with another treated for first-degree burns, the company said, adding that all workers were in stable condition.

Alberta Occupational Health and Safety has sent at least two officers to look for potential safety violations at the facility, the Canada Broadcasting Corporation (CBC) reported.