Two workers were killed and four others injured in an accident Tuesday afternoon at the company's Horizon oilsands construction site in northern Alberta. All the six workers were from China, Real Doucet, senior vice president of Oilsand, Canadian Natural Resources Ltd., told Xinhua in a telephone interview Wednesday.

The accident occurred when the roof collapsed on a massive oil container being built as part of the company's oilsands project north of Fort McMurray, Alberta, he said.

The four injured are non-critical and two have been released and the other two have been transferred to a hospital in provincial capital Edmonton, Doucet said.

Canadian Natural Resources Ltd. has started an investigation and a team is in place to determine the root cause of the accident,he said. Work at the tank has stopped and the area around the accident has been isolated.