A massive oil spill that occurred a week ago in Russia's Arctic region has resulted in an emergency declaration by the country's president and a government response to assess and clean up the damage.

Approximately 20,000 tons of diesel oil leaked from a storage tank on May 29, seeping into the Ambarnaya River in a part of Siberia located above the Arctic Circle, authorities said.

The tank was at an industrial plant operated by a subsidiary of Norilsk Nickel, the world's largest producer of palladium and one of the largest producers of nickel, platinum and copper.

The incident was reportedly caused by "abnormally mild temperatures" that caused the permafrost beneath the tank to thaw, leading to its structural collapse, according to a company official. A government investigation into the cause is being conducted. The French news agency AFP reports the spill has been contained.

The spillage was reported by the plant's employees two days after it happened, drawing criticism from Russian President Vladimir Putin.

"Why did government agencies find out about this only after two days?" Putin asked during a televised government conference on Wednesday. "Are we going to find out about emergencies from social media now?"

The Russian president then approved the state of emergency to manage the spill.

Government teams, including the Siberian branch of Russia's Ministry of Emergencies and Russia's Maritime & River Transport Agency, have been involved in cleanup efforts.

Vladimir Potanin, the billionaire president of Norilsk Nickel, said the company would pay the costs, estimated at $146 million.

The Investigative Committee of the Russian Federation is interrogating individuals about delayed warnings to the public about the spill.