European Commission President Jose Manuel Barroso said in a Wednesday letter to the Kremlin an interim solution to a trilateral gas crisis was mandatory.

Ukraine under an interim deal proposed during trilateral talks last week in Berlin would pay the $3.1 billion it owes Russian energy Gazprom. Ukraine, in return, would get a price discount and assurances of adequate winter natural gas supplies.

Barroso sent a letter to Russian President Vladimir Putin saying he expected a mutually acceptable deal should be reached by all parties to the negotiations.

"It is key that the resumption of energy deliveries to the citizens of Ukraine is ensured and that the fulfillment of all contractual obligations with customers in the European Union is secured," he said.

Russian Energy Minister Alexander Novak said Tuesday "it all depends on Ukraine."

Russian energy company Gazprom in 2006 and 2009 cut off gas supplies to Ukraine because of contractual disputes. With Ukraine's economy left in shambles following political upheaval in November, Gazprom switched its contract to a pre-payment scheme.

Another round of talks between Ukraine, the European Union and Russia is scheduled later this week in Berlin.