The President of the European Commission, Jose Manuel Barroso, Wednesday sent a letter to Russian President Vladimir Putin asking him not to interrupt gas supplies to Europe via Ukraine. Referring to recent discussions between the EU, Russia and Ukraine over gas prices supplies, he stressed that "it is imperative that all sides continue to engage in this process constructively and also agree on a future price that reflects market conditions". "As long as the trilateral talks are on-going, gas flows should not be interrupted. I count on the Russian Federation to maintain this commitment. 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," stated the letter. Barroso said the EU expects that all sides will "remain reliable and responsible supply and transit partners." "The European Union and other international donors have already provided significant support to Ukraine and will remain committed. I also expect a constructive approach from the Russian Federation to solve the issues at hand," added the letter. Meanwhile, Ukrainian Prime Minister Arseniy Yatsenyuk in a letter to EU leaders on Tuesday rejected Russia's demand to pay beforehand for its gas deliveries starting in June. "The issue of pre-payments cannot feature on the negotiating agenda" with Russia, he said. Russia's gas giant Gazprom warned last week that it will stop gas shipments to Ukraine on June 3 if Kiev does not start paying the gas bill debt of USD 3.5 billion . Russia delivers about 15 percent of all gas consumption in Europe via Ukraine