Russian and Ukrainian officials failed to reach an agreement early Tuesday on how much Ukraine should pay for Russian natural gas, a dispute that has exacerbated concerns about a possible supply interruption among European Union countries dependent on the gas for one-third of consumption.

The talks in Brussels were extended from last week to Monday evening and were presided over by the European Commission, the executive body of the EU. In negotiation is how much Ukraine should pay Russia for gas already consumed and for future consumption.

“We have some open questions and some different positions — ongoing different positions — but we agreed to continue to negotiate,” Günther Oettinger, the EU’s energy commissioner, said at a news conference Tuesday. “Tomorrow our Ukrainian partners and our Russian partners have to speak with their governments and their heads of states, and they are meeting to discuss the situation.”

The talks will continue Tuesday night or Wednesday morning, he said.

Tuesday’s meeting was the fifth between the energy ministers of Russia and Ukraine since May. In March, after Russia annexed Crimea from Ukraine, Russian state-controlled gas company Gazprom hiked gas prices by 80 percent on Ukraine and threatened to cut off supplies if the former Soviet satellite didn’t pay the new price.

Ukraine has already paid Russia about $786 million for gas delivered in February and March but must work out a repayment program for gas delivered late last year and work out a new price from April onward.