Russia's state-controlled natural-gas giant Gazprom has stopped deliveries of gas to Ukraine because of Kyiv's alleged failure to make the required prepayments.
Gazprom CEO Aleksei Miller said in Moscow on November 25 that no further gas deliveries will be forthcoming until payment is made.
He said Ukraine's refusal to buy Russian gas "creates risks for gas transit to Europe."
The gas cutoff comes less than two months after Russia and Ukraine signed an EU-brokered deal aimed at ensuring gas supplies through March.
Relations between Kyiv and Moscow have been soured since Russia annexed the Ukrainian Black Sea peninsula of Crimea in 2014 and began supporting separatist militants in eastern Ukraine.
Tensions have been particularly high in recent days after explosions damaged power lines sending electricity from mainland Ukraine to Crimea, plunging much of the peninsula into darkness.