About 1.3 million gallons of emulsion have been spilled in north-eastern Alberta, Canada, the Nexen oil firm has said.

The incident at the company's Long Lake oilsands project has affected an area of about 172,000 square feet along the pipeline's route.

Nexen said the spill of a mixture of bitumen, water and sand was discovered on Wednesday afternoon and has since been contained.

It is understood that the spill has not flowed into a body of water.

Alberta Energy Regulator and the company said it is too soon to say what might have caused the leak.

Nexen spokesman Kyle Glennie said the company is investigating how long the pipeline was leaking before it was shut off.

Alberta premier Rachel Notley said pipelines are still the best way to transport oil and gas despite this week's leak.

The company was taken over by China's CNOOC in 2013.