Ukraine has blocked the transit of Russian gas headed to western Europe in the Luhansk region, saying that the occupation of the eastern territory by Russian forces makes it impossible to oversee flows.
The decision means 32.6 million cubic metres of gas a day are to stop flowing at 7am local time on Wednesday.
That's about a third of what Russia exports across Ukrainian territory a day, according to Ukrainian regulators.
Ukraine said that the occupation made it impossible to access a node at Sokhranovka and direct the gas along other routes.
It cited at least one incident of "increased violence".
Gazprom, which supplies the gas that is sent through the pipelines, disputed the argument, saying it "had absolutely no confirmation of instances of increased violence".
It added that there is no technical way to reroute the affected gas along other routes.
Europe has been worried about its gas supplies ever since Russia's invasion of Ukraine began.
Many western European countries want to punish Russia economically for the invasion but have no alternative to the gas Russia supplies, meaning they are providing funding to a country against which they are also levying economic sanctions.
There have been worries since the war's start that Russia would turn off the gas to punish a continent that gets the bulk of the fuel from the country.
Until now, despite the war, the gas has continued to flow.
That said, much of Europe's gas supply from Russia comes via the Nord Stream 1 pipeline, which lies under the Baltic Sea.
According to contracts, a maximum of 109 million cubic metres per day is allowed to flow through pipelines criss-crossing Ukraine.
Given the uncertainties, European countries are searching for alternative fuel supplies.
For example, plans for the expedited construction of liquefied natural gas (LNG) terminals earned approval from the German cabinet on Tuesday.
The measure must now go to MPs and get approval in both the lower house Bundestag and the upper house Bundesrat if it is to become law.
The proposal is designed to make it easier to both floating and land-based LNG terminals, which would allow Germany to accept shipments brought in by boat from non-Russian gas producers.
However, to make that work, Germany needs the proper infrastructure.
The proposal allows regulators to skip processes in the interest of getting the terminals constructed faster, particularly in terms of environmental controls.
The hope is to have the first new LNG terminal operating by year's end.