Crisis-struck Ukraine has missed the payment deadline for its February gas supply from Russia's state-owned Gazprom.
The energy giant had already said it would scrap Ukraine's gas price discount after the revolution-gripped country warned it would not be able to pay its latest bill.
Alexey Miller, Gazprom chief executive, confirmed that Ukraine's payment deadline had passed. He said the bill is for $440m (£263m, €317m).
Russia had slashed gas prices for Ukraine in December, discounting the price per 1,000 cubic metres to $268.50 from $400, because the country was struggling to pay for the supply.
"Those who don't pay for goods supplied must understand they will face consequences and may lose shipments on easy terms," said Dmitry Medvedev, Russia's prime minister, in the days ahead of the deadline. Just over half of Gazprom is owned by the Russian state.
Ukraine is at the centre of a global political battle after a revolution ousted the country's Russia-allied president, Viktor Yanukovich.
Russian troops are on the ground in Ukraine's Crimea region, where Russia has many interests.
Vladimir Putin, Russia's president, insists this is to keep the peace amid the throes of revolution.
But the European Union (EU) and the US have said Russia's entry into Crimea is an act of aggression that breaches national sovereignty.
There is a tussle for influence over Ukraine between the west and Russia.
Russia seeks to defend its financial interest in Crimea, where it owns much of the industry and around 60% of the population is ethnically Russian.
However, many Ukrainians want to move closer to the EU and away from Russia, one of the reasons there was unrest and eventually a revolution.
Ukraine's parliament has just ratified a €610m loan from the EU, but it will need much more financial help if it is to plug a $70bn black hole in public finances identified by the new prime minister, Arseny Yatseniuk.
The US has offered an economic aid package that includes $1bn for energy. Russia offered Ukraine a loan of $2bn to $2bn to help clear its debt with Gazprom.
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