Suncor, Canada's largest oil and gas producer, shut the pipeline on Friday morning after discovering the spill at its main oil sands site near Fort McMurray in northern Alberta.

"The clean up continues and the pipeline remains shut in," said AER spokesman Jordan Fitzgerald. "At this time the cause of the incident remains unknown."

Fitzgerald said AER staff had visited the site and would be investigating the cause of the leak.

The pipeline is used for transporting diluent, an ultra-light hydrocarbon used to dilute viscous bitumen so it can flow, from Suncor's base plant to its Firebag plant for blending with bitumen.

Suncor said in an email on Monday it would not be providing daily updates on the repair and restart of the pipeline. On Friday, a spokeswoman said there had been no impact on operations.

(Reporting by Nia Williams; Editing by Marguerita Choy and Grant McCool)