Russia’s President Vladimir Putin has declared a state of emergency after 20,000 tonnes of diesel oil leaked into a river within the Arctic Circle.
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. It is not the first time Norilsk Nickel has been involved in oil spillages.
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.
Russian Minister for Emergencies Yevgeny Zinichev told Mr Putin that the Norilsk plant had spent two days trying to contain the spill, before alerting his ministry.
The leaked oil drifted some 12km (7.5 miles) from the accident site, turning long stretches of the Ambarnaya river crimson red.
The spill has contaminated a 350 sq km (135 sq mile) area, state media report.
In a statement, Norilsk Nickel said the incident had been reported in a “timely and proper” way.
The state of emergency means extra forces are going to the area to assist with the clean-up operation which could cost 100bn roubles (£1.2bn; $1.5bn) and take between five and 10 years.
The accident is believed to be the second-largest in modern Russian history in terms of volume, an expert from the World Wildlife Fund, Alexei Knizhnikov, told the AFP news agency.