Two workers at a giant oilsands project north of Fort McMurray were killed and four others injured when the roof of a massive storage tank collapsed on Tuesday, Alberta RCMP said.

All six workers involved in the accident were temporary labourers from China. The injuries to the four are not believed to be critical.

They were working on the storage tank at the Horizon Oil Sands Project about 70 kilometres north of Fort McMurray when the roof — which was higher than a three-storey building — fell on Tuesday afternoon.

The job site, an open pit mining project near the small community of Fort MacKay on the Athabasca River, is run by Canadian Natural Resources Ltd., an oil and gas company based in Alberta.

Alberta workplace health and safety officials are investigating. RCMP officers were at the scene until Tuesday evening.

Gil McGowan, president of the Alberta Federation of Labour, called Tuesday for a thorough investigation to ensure that all safety procedures were properly followed.

"We got phone calls from people on the worksite that this large metal vessel, which was under construction, collapsed inward and that there was a group of workers inside," he said.

"The people who phoned us to tell us about the accident had their cellphones confiscated so basically we're out of touch with them as well now."

As many as 400 temporary workers from China could be working on the project, McGowan added.

Alberta NDP Leader Brian Mason said he has questions about the kind of language training foreign workers have, as well as about the frequency of inspections on the job site.

Existing safety regulations in Alberta are adequate, but there are enforcement problems, said Mason.

"We're focusing on the fact that they don't have enough inspectors and they don't enforce the regulations that they do have. And that has contributed to unnecessary injuries and deaths."