May 30 (Reuters) - Canada has agreed to assess whether naphthenic acids found in northern Alberta's oil sands tailings ponds should be classed as toxic under federal law, a move that could pave the way for stricter regulations, environmental group Ecojustice said on Thursday.

Tailings - a mix of water, clay, sand and trace metals - are a byproduct of extracting bitumen from mined oil sands and are stored in huge engineered ponds, some of which have been accumulating water since the 1960s.

The ponds have come under increased scrutiny since last year, when it emerged wastewater had been leaking for months from a tailings pond at Imperial Oil's Kearl mine.

Ecojustice wrote to Canada's Environment Minister Steven Guilbeault in March formally requesting that the government study the risk that naphthenic acids from oil sands tailings pose to human health and the environment.

"Both the request and the information being collected and generated by the Government and others provide scientific evidence on the potential toxicity of OSPW NAs (oil sands process-affected water naphthenic acids) that warrants further investigation," Guilbeault responded in a letter dated May 28 that was shared by Ecojustice.

Imperial and Canada's two other oil sands mining companies, Suncor Energy and Canadian Natural Resources Ltd , did not immediately respond to requests for comment.

The chemical compound will be added to a plan prioritizing which substances should be assessed under the Canadian Environmental Protection Act, Guilbeault added. That plan will be finalised by June 2025.

The request was submitted by Ecojustice on behalf of the Athabasca Chipewyan First Nation, which lies downstream of the oil sands, and environmental groups. The nearby Mikisew Cree First Nation also submitted a similar request, Ecojustice said.

Naphthenic acids from tailings ponds have been found in surface water and groundwater in the region, said Ecojustice lawyer Bronwyn Roe.

"The harms that these chemicals cause to people and the environment need to be better understood, and NAs need to be regulated to prevent harmful exposure," Roe said in a news release.

Scientific studies have shown that naphthenic acids from oil sands tailings are toxic but the Canadian government has never formally classed them as such, said Alienor Rougeot, climate and energy program manager at Environmental Defence.

"If they are found to be toxic that would unlock a lot of regulatory tools," Rougeot added.

(Reporting by Nia Williams in British Columbia; editing by Diane Craft)