The spill happened when a fuel tank at a power plant near the Siberian city of Norilsk collapsed last Friday.

The power plant's director Vyacheslav Starostin has been taken into custody until 31 July, but not yet charged.

The plant is owned by a subsidiary of Norilsk Nickel, which is the world's leading nickel and palladium producer.

The Russian Investigative Committee (SK) has launched a criminal case over the pollution and alleged negligence, as there was reportedly a two-day delay in informing the Moscow authorities about the spill.

Ground subsidence beneath the fuel storage tanks is believed to have caused the spill. Arctic permafrost has been melting in exceptionally warm weather for this time of year.

President Putin expressed anger after discovering officials only learnt about the incident on Sunday.

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