The European Union called Wednesday on Russia to live up to its commitment to ensure continued gas supplies to Europe via Ukraine as long as talks on their future continued.
With Russian gas giant Gazprom warning it may halt shipments to Ukraine on June 3 unless its bills are paid, European Commission head Jose Manuel Barroso told Russian President Vladimir Putin it was "imperative" that negotiations continued.
The talks so far have established Kiev's "willingness ... to pay the agreed upon arrears," Barroso said in a letter to Putin.
Earlier this month, it had also been agreed that "as long as the trilateral talks are on-going, gas flows should not be interrupted," he noted.
"I count on the Russian Federation to maintain this commitment," Barroso said, adding that Gazprom accordingly must honour its supply contracts with European companies.
On May 14, Putin wrote a second letter to European leaders, warning that the European Union had so far made no concrete proposal to settle Ukraine's outstanding gas bills.
In an earlier letter in April, Putin had warned that without progress, gas supplies to Ukraine and onto Europe were at risk.
On Tuesday, Ukraine Prime Minister Arseniy Yatsenyuk rejected Gazprom's demand for up-front gas bill payments from June, saying it "cannot feature on the negotiating agenda."
Gazprom puts Ukraine's June bill at $1.66 billion (1.2 billion euros) and has previously said Kiev had accumulated $3.51 billion in debt.
The EU gets about 25 percent of its gas from Russia, with about half of that amount transiting through Ukraine.
As the Ukraine crisis has deepened, it has stoked fears Russia could cut gas supplies again, as in its 2006 and 2009 disputes with Kiev.
EU, Russian and Ukraine officials and ministers have held a series of meetings to try and settle the dispute and are expected to meet again on Monday in Berlin.