As crews work to clean up a massive pipeline spill in Alberta, local First Nations have been left to survey the damage to the land where they've lived for generations.

Over the weekend, crews worked to vacuum up 5 million litres of oil emulsion that seeped out of a breached Nexen Energy pipeline south of Fort McMurray.

It is unclear how and when the spill began, as the company's spill-monitoring system failed to spot the leak when it started.

It wasn't until Wednesday that a contractor noticed the mixture of sand, oil and water escaping from the pipe. By that time, enough emulsion had spilled to fill two Olympic-sized swimming pools.

Local indigenous people say the scale of the damage is devastating.

"It makes me feel sick," Robert Cree, a First Nations leader, told CTV Edmonton. "It hurts you."

Cree has spent most of his life living near Gregoire Lake, a short distance from where the spill occurred. He said he and his brother regularly use the surrounding land for fishing and hunting.

"I haven't even worked myself up to a point where I want to go see what (the spill) looks like," Cree's brother, Alden, said.

At the spill zone, Nexen crews have built a fence to keep wildlife away from the seepage, and are using a large hole to collect oil and pump it out. They say they plan to dig a second hole for the same purpose.

Nexen's senior vice-president of Canadian operations, Ron Bailey, apologized for the spill on Friday, and said the company will look into why the automated-monitoring system didn't detect it.

The company said it also plans to investigate the full extent of the environmental damage.

In his remarks on Friday, however, Bailey said the leak did not immediately affect any people.

Noting that the closest homes to the spill are 15 kilometres away, in the community of Anzac, Bailey said: "There's no residences here, so there was no impact to that.

"There's no human impact here, immediately."

For the Cree brothers, Bailey's assessment is wrong. They say the spill does affect people in their communities and families.

Local band councillor Byron Bates said he supports "responsible" oil extraction, but his main concern is keeping the land safe for generations to come.

"What we care about is if the land's still going to be useable for our purposes in 50 years, when all the oil companies are gone," he said.

With files from CTV Edmonton's Josh Skurnik and the Canadian Press