One of the largest leaks in Alberta history has spilled about five million litres of emulsion from a Nexen Energy pipeline at the company's Long Lake oilsands facility south of Fort McMurray.

The leak was discovered Wednesday afternoon.

Nexen said in a statement its emergency response plan has been activated and personnel were onsite. The leak has been stabilized, the company said.

The spill covered an area of about 16,000 square metres, mostly within the pipeline corridor, the company said. Emulsion is a mixture of bitumen, water and sand.

The pipeline that leaked is called a "feeder" and runs from a wellhead to the processing plant.

"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 in the statement issued Thursday.

Peter Murchland, public affairs manager for the Alberta Energy Regulator, said officials were notified late Wednesday and had staff onsite Thursday to work with Nexen.

"My understanding is that the pipeline and pad site had been isolated and shut-in earlier today, effectively stopping the source of the release," Murchland said

Nexen has contained the leak and started cleaning up the area, he said. There was no word on how long that might take.

"They go through a cleanup phase in accordance with the regulations set by the AER," he said. "And we'll have our subject-matter experts work alongside the operator, today and going forward, to make sure that safety and environmental requirements are met."

The regulator's staff are there to oversee the company's cleanup efforts. Murchland said there have been no reports about any effect on wildlife. The regulator has ordered the company to implement a wildlife protection plan.

Premiers talks focus on energy, pipelines

Greenpeace issued a statement Thursday condemning Alberta's history of pipeline spills.

"As provincial premiers talk about ways to streamline the approval process for new tar sands pipelines, we have a stark reminder of how dangerous they can be," Greenpeace said in a news release.

Canada's premiers are meeting in St. John's, where a major topic of discussion is a national energy strategy.

"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," said Greenpeace communications officer Peter Louwe. "New pipelines would also facilitate the expansion of the tarsands — Canada's fastest-growing source of carbon emissions — and accelerate the climate crisis even more.

"We need to stop new pipeline projects before they're built and focus on building renewable sources of energy that are sustainable and won't threaten communities, our environment, and the planet."

In April 2011, a Plains Midstream Canada ULC pipeline leaked 4.5 million litres of crude oil near a First Nations community in northwest Alberta.

That leak was the largest in the province in 35 years. It contaminated more than three hectares of beaver ponds and muskeg in a densely forested area.