Nexen Energy apologized Friday for an oil sands pipeline leak in the Canadian province of Alberta that is one of North America's largest-ever oil-related spills on land, and said its cleanup crews were working around the clock. The pipeline leaked 31,500 barrels of emulsion — a mixture of bitumen, water and sand, the equivalent of 5 million liters of tar sands oil, or 1.3 million gallons.

Nexen, a subsidiary of China's CNOOC Ltd., said it is still trying to find the root cause of the leak in the new pipeline, which was installed last year. The company has found a visible breach about the size of a hand.

The incident is another blow for the environmental record of the oil sands industry, already under fire from activists for its carbon-intensive production process.

Ron Bailey, a Nexen senior vice president who leads the company's Canadian operations, said the company is "deeply concerned" about the spill, which it detected at its Long Lake facility on Wednesday.

The company said it thinks restarting the pipeline, which connects its Kinosis oil sands project to the Long Lake facility, will take "some time."

The spill covered about 4 acres, and the Alberta Energy Regulator (AER) said the leak did not contaminate any bodies of water.

The regulator has sent investigators to Long Lake to try to determine the cause of the pipeline failure.

Environment Canada's Enforcement Branch said it has opened a file on the incident, and was in the early stages of gathering information.

"This leak is also 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," Greenpeace spokesman Peter Louwe told CBC. "New pipelines would also facilitate the expansion of the tar sands — Canada's fastest-growing source of carbon emissions — and accelerate the climate crisis even more."

Nexen shut down the pipeline at its 72,000-barrel-per-day Long Lake facility, about 22 miles southeast of the oil sands hub of Fort McMurray, and said it has isolated the pipeline as part of the clean up operation.

Al Jazeera and Reuters