Ukrainian authorities have stopped the flow of Russian natural gas through one key hub that feeds European homes and industries.
Ukraine’s natural gas pipeline operator (GTSOU) said it would stop shipments through an eastern part of the country due to interference from “occupying forces”.
Russian-backed separatists who control the region in eastern Ukraine have been accused of siphoning off gas at the Sokhranovka transit point.
It is the first time since the start of the war that Kyiv has symbolically disrupted the flow of Russian energy exports to the west.
The Sokhranovka transit point usually handles around a third of Russian gas flows to Europe -- mostly to Austria, Italy, Slovakia and other east European states.
The head of GTSOU said on Facebook that Moscow must bear full responsibility for any humanitarian consequences of the halt of gas to Europe.
The immediate impact of the cutoff was likely to be limited since much of the gas can be directed through another pipeline, according to analyst Zongqiang Luo at Rystad Energy.
Luo told the Associated Press that the move by Ukraine would make it harder for European countries to refill underground storage for next winter.
The interruption of supply would also “hasten Europe’s plans to move away from imports of Russian gas,” he added.
The European Commission said on Wednesday that the sudden interruption would have an impact on part of the bloc's gas supply but that there were no "immediate security of supply concerns".
EU Energy Commissioner Kadri Simson is due to speak to Ukrainian Energy Minister German Galushchenko.
Russia’s state-owned giant Gazprom has indicated some drop in supply reaching Europe but stated that it was continuing to meet its obligations.
Gazprom said on Wednesday it was sending 72 million cubic metres of gas through Ukraine, down around 25% from the previous day.
The European Union has sought to reduce its dependence on Russian energy, phasing out its use of coal and considering doing the same for oil. Gas presents a more complicated problem, given both how much Europe uses and the technical difficulties in sourcing it elsewhere.
Gazprom had already decided to cut off supplies to Poland and Bulgaria after the two countries refused to pay in roubles.
It was not clear if Russia would take any immediate hit since it has long-term contracts and other ways of transporting gas.