Russian Deputy Energy Minister Anatoly Yanovski said Russia could send more natural gas through pipelines that avoid geopolitically sensitive territory in Ukraine, like the Nord Stream pipeline through the Baltic Sea. Some Gazprom customers, he said, have few other options, however.
"Countries that have no alternative supply [channels] other than the Ukrainian gas transport system are most at risk," he said Monday.
Yanovski said Bulgaria, along with Bosnia and Herzegovina, were among those with the most risk.
European consumers get about a quarter of their gas needs meet by Russia, though most of that supply runs through the Soviet-era transit network in Ukraine.
Yanovski says Ukraine has few other options apart from paying its mounting debt to Gazprom. The Russian energy company in 2009 cut gas supplies through Ukraine in response to a similar gas row.
Gazprom in April sent its Ukrainian counterpart Naftogaz a bill for more than $11 billion for taking on less gas than contracted in 2013.