Russia warned Europe that gas could stop: Slovak PM

Dmitry Kostyukov | AFP | Getty Images

Russian President Vladimir Putin has informed multiple European states that Moscow will not supply gas to Europe as of June 1 if Ukraine does not pay its bills, Slovak Prime Minister Robert Fico said on Thursday.

"Today, multiple member states including Slovakia were informed by President Putin that as of June 1, if Ukraine does not pay for supplied gas, it will not be supplied to the European territory," Fico said.

Dmitry Kostyukov | AFP | Getty Images

Russia's Gazprom says low risk to Europe supplies

"It is a serious threat to the energy security of Slovakia."

Earlier on Thursday, Putin said Russian gas exporter Gazprom had been forced to demand Ukraine pay in advance for gas as of June after its debt for gas already delivered reached $3.5 billion.

Moscow has reduced gas supplies to Kiev during two past price disputes, causing disruptions to supplies to Europe.

Read MoreUkraine crisis hardens Brussels stance on Gazprom pipeline

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