Ukraine raised the stakes in a dispute with Russia over gas supplies on Tuesday, saying Russian state-controlled company Gazprom owed Kiev natural gas worth around $1 billion which it had "stolen" when Moscow annexed Crimea.
Russia has warned it will reduce gas supplies to Ukraine on June 3 if Kiev fails to pay in advance for next month's deliveries, causing concerns that onward supplies to Europe could be threatened.
With newfound confidence inspired by the election of a Ukrainian president at the weekend, Prime Minister Arseny Yatseniuk said gas talks with Russia could not progress until he heard Moscow's response on giving back the 2.2 billion cubic metres of gas which he said was taken when the country's Black Sea region was annexed by Russia in March.
The dispute has strained ties between the two neighbours since Moscow almost doubled the price for its deliveries to Ukraine after protesters toppled a pro-Russia Ukrainian president.
It has also renewed concern in the European Union that there could be disruptions to Russian supplies delivered through pipelines that cross Ukraine.
"We want to hear a response from Russia ... on the question of returning to Ukraine 2.2 billion cubic metres of gas which Russia stole through Chernomornaftogaz on the territory of (Crimea)," Yatseniuk told a televised session of his cabinet.
He did not explain the source of that figure but Kiev had referred earlier to a similar figure held in storage in Crimea.
Yatseniuk also said if there was no agreement between the two sides by May 29, Ukraine would next meet Gazprom at the Stockholm arbitration court, which would try to resolve the dispute.
He reiterated that Kiev would pay off its debts when there was an agreement on price.
Asked about the accusation that Moscow had stolen gas, Gazprom spokesman Sergei Kupriyanov said: "We have no idea what he means."
He added that as far as Moscow was concerned, both sides had come up with a plan of action at EU-brokered talks in Berlin: Ukraine would pay Russia $2 billion of its debts by the end of the week, and $500 million by June 7 for May deliveries.
"This position seemed to be the final decision of the three-way meeting," he said.
Gazprom CEO Alexei Miller told Rossiya-24 television late on Monday that the company was ready to provide a discounted gas price for Ukraine but only after Kiev paid at least