Diesel fuel from a 21,000-ton oil spill in the Russian Arctic has reached a freshwater lake that serves as a gateway into the Arctic Ocean, Russian officials announced. The spill, one of the largest in Russian history, has contaminated several rivers and tributaries so far, triggering a major clean-up effort and prompting Russian President Vladimir Putin to declare a state of emergency.

The spill, which occurred near the city of Norilsk on May 29, originated from a fuel reservoir at a power plant owned by the metals company Norilsk Nickel, the world’s largest producer of nickel and palladium. The company said the incident happened as a result of thawing permafrost, which weakened the foundations for the storage tank. But several environmental groups, including the World Wildlife Fund and Greenpeace Russia, said that while global warming likely played a role, the company also has a track record for neglecting its aging infrastructure, The Guardian reported. On a televised call last week, President Putin told Norilsk Nickel’s president, Vladimir Potanin, "if you had changed them [the company’s aging reservoirs] on time, there would have been no environmental damage and no need to foot such costs."

Governments officials said the oil has now spread as far as Lake Pyasino, located 12 miles north of Norilsk, Reuters reported. The 45-mile-long lake feeds into the Pyasina River, which flows into the Kara Sea in the Arctic Ocean. A spokesperson for Norilsk Nickel denied, however, that the oil had reached the lake and said there was no risk of it flowing into the Arctic Ocean.

"This will have a negative effect on the water resources, on the animals that drink that water, on the plants growing on the banks," Vasily Yablokov, Greenpeace Russia’s climate project manager, told Reuters.