Moscow announced a 20% cut in the volume gas piped to Europe via Ukraine, which is refusing to pay higher prices demanded by Russia's monopoly supplier Gazprom.
Although Gazprom's boss promised to make up the EU shortfall by sending more gas via other routes, the decision increased concern in Brussels of energy shortages as winter bites.
Moscow's announcement came after senior EU diplomats had been meeting in Brussels to hear that the dispute has already led to significant gas supply reductions in six mid-European member states - Hungary, Poland, Romania, Greece, Bulgaria and the Czech Republic.
Moscow blames Ukraine, which Russia accuses of siphoning off supplies destined for the EU to make up for last week's Gazprom decision to cut off gas to the country.
Ukraine denies the charge and says technical problems are responsible.
But Moscow claims its 20% cut is equivalent to the volume of gas Ukraine is taking from the pipeline system.
At talks in Brussels, some mid-European member states said they had recorded drops in gas supplies in the last five days - since the dispute with the Ukraine began.
But so far the knock-on effect has been limited and diplomats decided to take no further action, pending the outcome of a current EU delegation visit to Kiev to size up the prospects of a swift settlement of what the EU says is a "commercial" dispute.