Russia supplies about a quarter of Europe's gas, with just over half of that flowing through Ukraine.
The ability for Russia to cut those gas supplies provides potential leverage as Western states threaten to impose economic sanctions over its actions in Ukraine.
"What sanctions can you place on a country that can cut off your gas?" one unnamed Italian official told the Economist.
There is history here too: during disputes in both 2006 and 2009, Russia cut off gas supplies to Ukraine. However, energy experts have talked down the prospect of a repeat.
"With everything else going on, I think it would only be in the context of if things somehow got much worse," energy historian Daniel Yergin said.
"From the viewpoint of Russia, part of its value-proposition is that it's a reliable supplier."
The United States, meanwhile, is reportedly moving to reduce Ukraine's dependence on Russian gas.