European Commission José Manuel Barroso sent a letter Wednesday to Russian President Vladimir Putin. In it, he said it was important that all parties to a row over gas supplies through Ukraine remain engaged.
"As long as the trilateral talks are on-going, gas flows should not be interrupted," the letter stated. "It therefore continues to be Gazprom's responsibility to ensure the deliveries of the required volumes as agreed in the supply contracts with European companies."
Gazprom in April sent its Ukrainian counterpart Naftogaz a bill for more than $11 billion for taking on less gas than contracted in 2013. Moscow has called on Kiev to pay in advance for natural gas in order to avoid a similar row in 2009 that resulted in a decision by Gazprom to cut off gas through Ukraine.
Barroso said a "structural solution" requires an open and transparent Ukrainian gas sector with Ukraine doing its part to ensure secure gas flows.
"It is the European Union's clear expectation that all sides will remain reliable and responsible supply and transit partners, which is also very much in their own interest," he said.
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.