Hungary has stopped transporting natural gas to Ukraine, according to Ukraine's state-run oil and gas company Naftogaz.
Deliveries were halted on Thursday evening, with the Hungarian company saying supplies would be cut off due to to technical reasons and would remain suspended "until further notice", Naftogaz said in an emailed statement.
The decision to cut off reverse gas supplies to Ukraine was revealed on the same morning that Russia's energy minister warned European countries against re-exporting gas to Ukraine.
Russia's Energy Minister Alexander Novak has told a German newspaper that EU countries that re-export gas to Ukraine would face supply cut-offs from Russia.
"The contracts signed [between Russia and EU clients] do not have any provisions for re-exports... We hope that our European partners respect the past agreements. That is the only way to guarantee uninterrupted supplies," he said, as quoted by the Handelsblatt newspaper.
Ukraine, Russia and the European Union are due to meet today (26 September) in a bid to resolve the standoff between Kiev and Moscow over the price Ukraine pays for natural gas.
Russia almost doubled the price it charges Ukraine for gas days after it annexed the Crimea peninsula from its neighbour in March.
Kiev had refused to pay the new price, prompting Russian state-run gas firm Gazprom to cut off gas supplies to Kiev in June.
Ukraine has since boosted imports of gas from some EU countries and has begun eating into its own supplies to make up for the shortfall in Russian gas.