A pipeline leak in western Canada has spilled some 31,500 barrels (five million liters) of oil sands, the owner of the affected facility said on Friday.

A mixture of bitumen, waste water and sand began oozing on Wednesday from the ruptured pipeline at Nexen Energy's Long Lake oil sands site in Alberta, the company said.

Clean-up operations are underway, it added.

"All necessary steps and precautions have been taken, and Nexen will continue to utilize all its resources to protect the health and safety of our employees, contractors, the public and the environment, and to contain and clean up the spill," the company said.

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

Greenpeace, meanwhile, said the spill was 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.

"This leak is ... a good reminder that Alberta has a long way to go to address its pipeline problems, and that communities have good reasons to fear having more built," said Greenpeace communications officer Peter Louwe.