Russia's Gazprom could restrict gas supplies to Ukraine if Kiev fails to make a prepayment by the end of May, Russian Energy Minister Alexander Novak said on Friday.

"If that invoice will not be paid for June by the 31st of May, Gazprom actually will have the possibility, in accordance with the provisions of the contract, to restrict the supplies of gas to Ukraine or to guarantee the supply of gas only in the volumes for which the invoice will be paid," he told reporters in the Polish capital Warsaw.

Novak's comments come after trilateral talks over Russia's gas supplies to Ukraine and Europe with Ukrainian counterpart Yuri Prodan and EU energy commissioner Guenther Oettinger.

The talks were held just days after the United States and the European Union slapped fresh sanctions on Moscow amid escalating violence in Ukraine's eastern regions.

Russia accounts for about a quarter of all EU gas supplies while Ukraine's pipelines carry about half of those imports each year, according to EU data.

The three sides "agreed to convene the next meeting in mid-May, with a view to find a solution by the end of May" about future gas supplies, the European Commission said in a statement.

"It has been confirmed by all sides that, as long as trilateral talks are on-going, gas flows will not be interrupted."

In April, Russian President Vladimir Putin sent a letter to 18 EU countries warning that Moscow could cut gas supplies unless they helped Ukraine pay its outstanding bill.

Russia made a similar move in 2009 when supplies to Europe moving through Ukraine were badly disrupted.

Novak said Friday "the general debt for the supplied gas to Ukraine in accordance with the current contract that has been binding since 2009 is about $3.5 billion (2.5 billion euros)."

"This is a debt that increased in April by $1.3 billion."

Ukraine's crisis erupted after pro-EU protestors in Kiev toppled Russian-backed president Viktor Yanukovych, but has rapidly degenerated into a full-blown global crisis.

In response, EU leaders have stressed the need to reduce Europe's dependence on Russian gas to reduce Moscow's leverage, especially in eastern Europe.

On Monday, Slovakia and Ukraine signed a deal to give Kiev access to European gas supplies in a "reverse flow" arrangement that would give Ukraine an alternative source of energy.

Novak said Friday that "it is possible that (the deal) would not be legal."

"It would be possible that the reception of the Russian gas that is devoted to European consumers would not be legal to send it to Ukraine.

"If such contracts are executed, we'll very attentively look at them and we leave ourselves the right to consider these questions and address various institutions of arbitration," he concluded.