The Athabasca Chipewyan First Nation (ACFN) is demanding answers and action from the province following reports of a large petrochemical spill in the Athabasca River.

A member of the ACFN first reported seeing a large oily sheen on the river about 60 kilometres north of Fort McMurray on Saturday morning.

By the afternoon, the sheen had grown to over 100 kilometres in length, said Chief Allan Adam in a release made Sunday.

Believing the sheen to be the result of a petrochemical leak, the ACFN says it reported the problem to the province’s energy regulator and to Environment and Sustainable Resource Development.

"Our Nation faces another toxic threat to our water supply and our calls for action are met by silence by the Alberta government and their new energy regulator," said Chief Adam in the release. "Our members appear to be the only world class monitoring system Alberta has."

Adam added that members of the ACFN have been documenting the river sheen and collecting water samples.

However, a spokeswoman for the province says it can find no signs of a petrochemical spill on the river.

Robyn Cochrane is the communications director for Environment and Sustainable Resource Development. She told CBC News that officials found no sign of hydrocarbons in the water.

"We had regional responders the length of the Athabasca River from Fort McMurray to Fort Chipewyan. They found no obvious source of hydrocarbon leaks," she said, adding that one sandbar appeared to be slightly dark, but was found to be natural upon closer inspection.

"[The province also] asked major industry in the area to check their facilities to ensure there that there’d been no spills or leaks," she added. "Industry completed a thorough check of the facilities — no sources were found."

Officials also flew over the river but could find no obvious signs of a leak, she added.