Russian gas giant Gazprom said on Tuesday that Ukraine had until June 2 to pay $1.6 billion (1.2 billion euros) for natural gas or Moscow will turn off its energy supplies.
Gazprom spokesman Sergei Kupriyanov said that the Russian firm had given Ukraine's Naftogaz energy firm a preliminary June bill based on the contract volume of 114 million cubic metres of natural gas per day.
"Naftogaz will have to pay this bill before June 2," Kupriyanov said in a statement. "Starting from June 3, the company will receive gas for Ukrainian consumers only in the amounts it has paid for."
Gazprom currently charges Ukraine $485.5 per 1,000 cubic metres of gas. According to the agreement, the June bill would total $1.66 billion for 3.4 billion cubic metres of natural gas.
Gazprom has said it was switching to a new payment system which will require Ukraine to cover its gas supplies upfront, because the Ukrainian firm has amassed a debt totalling $3.51 billion.
Kiev authorities counter that Russia is punishing Ukraine for its pro-Western aspirations and have threatened to take Moscow to court.
In April, Russian President Vladimir Putin sent a letter to 18 European Union countries warning them that gas supplies could be interrupted and urging them to help pay Ukraine's debts.