Russian state company Gazprom says Ukraine has to pay upfront for its gas supplies, after Kiev failed to settle its huge debt.
Gazprom had warned it would cut supplies if Ukraine failed to pay $1.95 billion – out of $4.5 billion it says it is owed – by 06:00 GMT.
There is no confirmation supplies have been ended. The Russian firm said it would continue to supply gas to Europe.
Russia-Ukraine ties remain tense since Moscow annexed Crimea in February.
Kiev accuses Moscow of supporting separatists in the east of the country. Russia denies the charge.
“Today, from 10:00 Moscow time, Gazprom, according to the existing contract, moved Naftogaz [Ukraine’s state-owned company] to prepayment for gas supplies,” Gazprom said in a statement.
“From today, the Ukrainian company will receive Russian natural gas only in the amounts it has paid for.”
The move follows crisis talks between Ukraine, Russia and the EU on the issue.
“We reached no agreement. The chances that we meet again are slim,” Gazprom spokesman Sergei Kuprianov said after the latest round of the talks ended in Kiev over the weekend.
However, EU Energy Commissioner Guenther Oettinger, who attended the talks, said he was “not pessimistic” about a deal.
He said he would continue to work for an agreement despite his compromise proposal, that Kiev pay $1 billionn on Monday and the rest in installments, being rejected by Gazprom.
Ukraine’s discounted rate for gas was axed in April after Moscow accused Kiev of failing to pays its bills.
On Monday, Gazprom stressed that it would continue to supply European consumers with gas at “full volume” and that it was Ukraine’s responsibility to make sure the gas transited through the country.
However, correspondents say the EU could be affected.
Earlier this month, Gazprom gave Ukraine more time to settle its gas bill after receiving a part-payment of $786 million.
Ukraine said it refused to clear its debts completely in protest at Gazprom’s recent 80% price increase.
Gazprom ended its discount price for Ukraine, which was negotiated by former President Viktor Yanukovych last December, in April.
Before the discount was cancelled, Ukraine’s gas bill was heavily reduced by Russia to $268 per 1,000 cubic meters.
The price is now $485.50 per 1,000 cubic meters, the highest in Europe.
Almost 15% of gas used in Europe comes from Russia via Ukraine, which is why EU members are taking a particularly close interest in the stand-off, observers say.
The talks that ended on Monday had been brokered by EU representatives.
Heading into the negotiations, Kiev said it was ready to make the $1.95 billion payment if Russia cut its price to $326 per 1,000 cubic meters.
But Russian President Vladimir Putin said $385 per 1,000 cubic meters was his final offer.
Prospects of a breakthrough in discussions were diminished in recent days after increasing tension between Kiev and Moscow over the pro-Russian insurgency in the east of Ukraine.
Moscow reacted angrily to a violent protest outside its embassy in Kiev on Saturday after separatists shot down a Ukrainian military plane, killing all 49 people on board.
Meanwhile, Ukraine accused Russia of sending tanks into the east of the country to support pro-Russian fighters.
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