Russian energy giant Gazprom on Friday warned Ukraine it could cut off gas exports if the new authorities in Kiev did not pay a bill for debt that now stands at $1.89 billion.
"Ukraine has de-facto stopped paying for gas... We cannot deliver gas for free. Either Ukraine pays the debt and pays for current supplies or the risk appears of a return to the situation at the start of 2009," said Gazprom chief executive Alexei Miller, quoted by Russian news agencies.
Gazprom in 2009 cut off gas to Ukraine in a move that also left much of the EU without supplies. Europe imports around a third of its total gas needs from Russia.
Miller said that Friday was the final cut-off date for payments for February's gas deliveries to Ukraine.
"Gazprom has not received the payment on its account. The debt due has increased and is now $1.89 billion (1.36 billion euros)," he added.
Turning off gas to Ukraine would punish the new Ukrainian government that took over after the February 22 ouster of president Viktor Yanukovych, which Russia has so far refused to recognise.
But it would also hit Europe, which cheered the fall of Yanukovych and relies on Ukraine for the transit of much of the gas that it receives from Russia.
Gazprom has already said it will end the substantial discount that Ukraine receives on market gas prices from April. But the European Union has said it will help Ukraine pay its bills.
The 2009 dispute -- which left homes in some European countries unheated in a freezing winter and also forced cuts in supplies to industrial customers -- was brought to an end in a deal between Russian strongman Vladimir Putin and then Ukrainian premier Yulia Tymoshenko.
Tymoshenko was later jailed under Yanukovych's rule for abuse of power because of her role overseeing the signing of the deal, which opponents said had been a bad one for Ukraine.
Gazprom's threat caused an immediate selloff in Europe's main stock markets which had briefly ticked into positive territory over positive US employment data.
Frankfurt's DAX 30 tumbled 2.01 percent to close at 9,350.75 points, while London's FTSE 100 dropped 1.12 percent to 6,712.67 points and in Paris the CAC 40 fell 1.15 percent to 4,366.42 points.
Industry experts say thanks to the mild winter European countries have about six weeks of reserves and as warmer spring weather arrives demand should fall.
Nevertheless Bulgaria, one of the countries worst affected by the 2009 cuts as it is 90-percent dependent on gas shipped through Ukraine, announced earlier this week it is stockpiling supplies.