A pipeline at Suncor Energy's oil sands base plant remained offline on July 25, the Alberta Energy Regulator (AER) said, following a leak the week of July 18 that spilled about 125 barrels (bbl) of diluent.

Suncor, Canada's largest oil and gas producer, shut the pipeline on the morning of July 22 after discovering the spill at its main oil sands site near Fort McMurray in northern Alberta.

"The cleanup 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 ultralight hydrocarbon used to dilute viscous bitumen so it can flow, from Suncor's base plant to its Firebag plant for blending with bitumen.

Suncor did not immediately respond to requests for comment, but said on July 22 there had been no impact on operations.