A decades-long arrangement allowing Russian gas to flow to EU states via Ukraine has ended after a five-year deal expired.
Ukrainian President Volodymyr Zelensky said his country would not allow Russia to "earn additional billions on our blood" and had given the EU a year to prepare.
Russia can still send gas to Hungary, Turkey and Serbia through the TurkStream pipeline across the Black Sea.
The European Commission said the continent’s gas system was "resilient and flexible" and that it could cope with the change. Slovakia, however, could face disruption and Moldova, which is not in the EU, is already suffering shortages.
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