Russian metals giant Norilsk Nickel on Monday admitted a spillage at one of its plants was responsible for turning a local river blood-red.

Russia's environment ministry last week launched a probe into the incident after images showed the Daldykan river near Norilsk in the far north of Russia flowing bright red, with local activists blaming the nearby Nadezhda Metallurgical Plant.

After initially refusing to confirm a leak, Norilsk Nickel -- the world's biggest producer of nickel and palladium -- on Monday said heavy rains on September 5 had resulted in water flooding over a "filtration dam" at the plant and into the river.

"Despite the short-term discolouration of the water...this incident does not present a danger for people or fauna in the river," the company said in a statement.

Environmental activists, however, insisted that it was too early to judge the environmental impact as the official investigation was still ongoing.

"You can't just say that it's no big deal. Right now there is a ministry of environment commission there," Russian Greenpeace official Alexei Kiselyov told AFP.

Kiselyov said that investigating pollution from Norilsk Nickel plants was extremely difficult as its infrastructure was located in remote areas and the firm controlled access to the entire Taymyr peninsula, which lies between the Kara and Laptev seas in the Russian Arctic.

Groups representing indigenous populations in the area say that local media went out of their way to whitewash the company and did not inform the public after the accident.

"We had a report after it happened that claimed the river colour came (naturally) from clay. That is just laughable to local people," said Sidor Chuprin, an indigenous activist.

He accused Norilsk Nickel workers of poor safety standards and said locals are concerned about the Daldykan river because they fish in another river further downstream.

"They don't care about polluting because they all have homes on the mainland," he told AFP, referring to central Russia.

Chuprin said that there was no environmental officials based on the remote Taymyr peninsula and locals struggled to hold the metals giant to account.

"Of course this is in their interest," he said of Norilsk Nickel. "We are not experts, all we can do is take a picture."

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NORILSK NICKEL