The oil and gas producer Nexen Energy, a Canadian subsidiary of China's CNOOC, was forced to suspend operations Saturday at 95 pipelines after a major leak last month.

The leak in Alberta, western Canada, spilled some 31,500 barrels (five million liters) of oil sands, prompting concern and criticism from environmental groups.

The Alberta Energy Regulator (AER) ordered the immediate suspension after it accused Nexen of "noncompliant activities" at the firm's Long Lake oil-sands operations in terms of maintenance and monitoring.

Nexen will have to provide documentation to assure the agency that it can operate the pipelines safely.

"Protection of public safety and the environment are the AER’s top priority," said Jim Ellis, AER president and CEO.

"Given that this company has already had a pipeline failure at this site, the AER will not lift this suspension until Nexen can demonstrate that they can be operated safely and within all regulatory requirements.

"We will accept no less than concrete evidence."

Nexen had said previously that no injuries occurred as a result of the spill and that the problem pipeline had been "isolated."

Greenpeace says the spill is just the latest evidence of environmental risks posed by the controversial practice of extracting oil from tar sands.

Critics blame massive growth in the Alberta oil sands for a spike in Canadian CO2 emissions that have contributed to Canada's failure to meet its international obligations to curb global warming.