Ron Bailey, the company's senior vice-president of Canadian operations, said it is investigating what caused the pipeline to rupture and why the system failed.
"We are deeply concerned with this and we sincerely apologize for the impact that this has caused," Bailey told a news conference.
"We will take every step that we see as reasonable and as the regulators help us decide what to do to respond to this."
A contractor discovered the leak Wednesday near Nexen's Long Lake oilsands facility about 35 kilometres southeast of Fort McMurray, Alta. Nexen shut down the pipeline, but not before some five million litres of bitumen, produced water and sand spilled into muskeg.
Nexen, which was taken over by China's CNOOC Ltd. in 2013, says the affected area is about 16,000 square metres, mostly along the pipeline's route.
Bailey said company staff have been on site to contain the spill and prevent impact on wildlife and a nearby lake. He said the company built an all-weather road to bring in trucks to vacuum the spill.
Alberta Premier Rachel Notley, who was in St. John's, N.L., on Friday for the annual premiers' meeting, said she has been receiving updates on the spill.
Notley said pipelines continue to be the safest way to carry oil and gas across the country.
"So what it comes down to is we need to learn from the spill," she said.
A spokesman for Energy Minister Margaret McCuaig-Boyd said she is in regular contact with the Alberta Energy Regulator and is monitoring the situation closely.
"We take a pipeline spill like this one very seriously, " Brad Hartle said in a statement.
John Bennett, national program director of the Sierra Club Canada Foundation, said he was worried.
"We're always concerned when petroleum products get spilled into the environment. There's always damage, and it's usually permanent of some nature," said Bennett. "It's full of toxic elements that should not be released into the environment."
In March, Murphy Oil spilled about 2.7 million litres of condensate at an oilfield in northwestern Alberta. Condensate is used to dilute heavy oil so it can flow through pipelines.
In 2011, about 4.5 million litres of oil leaked from a Plains Midstream pipeline into marshlands near the northern Alberta community of Little Buffalo.
Melina Laboucan-Massimo, a Greenpeace campaigner from Little Buffalo, said she was disappointed to hear of another spill.
She said she still remembers the impact the Plains spill had on her family.
"My family had been calling and texting me saying their eyes were burning, their stomachs were turning, they were feeling noxious, they couldn't breath," she said. "That's what happens when big, five million litre spills like this happen."
Bailey said there are no residences near this most recent spill, with the closest community, the hamlet of Anzac, sitting about 15 km north.
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