President Vladimir Putin says that, starting in June, Russia will deliver gas to Ukraine only if it pays in advance.
The warning came in an open letter to European leaders released by the Kremlin on May 15, Radio Liberty reports.
Putin said state-controlled Gazprom has been forced to demand advance payments after Ukraine's debt reached $3.5 billion.
Putin also called on the European Union to "more actively engage" in finding ways to stabilize Ukraine's crisis-hit economy.
Putin first warned Russia will demand that Ukraine pay in advance for Russian gas in a letter to European leaders last month.
Ukraine has said it could start paying off its debt if Moscow restored the lower gas price Kyiv was paying before pro-Russian President Viktor Yanukovych was ousted in February.
Gazprom more than doubled the price of gas for Ukraine after Yanukovych was ousted.