Last ditch talks between Russia and Ukraine have failed to resolve their long running gas pricing dispute.
Moscow has set 8 am Central European Time on June 16 as the deadline by which Kyiv must pay nearly two billion dollars of debts.
Cutting supplies could disrupt the gas flow to the European Union which gets some of its imports via Ukraine.
The two countries disagree on how much Kyiv should pay for the natural gas it receives from Russia.
Moscow increased its tariff after the ousting of Ukrainian president Viktor Yanukovych.
Kyiv wants to pay the previously agreed amount. A compromise proposal put forward by the EU has been rejected by Moscow.
Moscow now says it wants Ukraine to pay for its gas in advance.
