President Vladimir Putin said on Wednesday that he did not want a gas conflict with Ukraine similar to ones that took place in the past, and that Kiev only had enough pre-paid gas from Russia to last two days.
Moscow cut off gas supplies to Kiev from June until December in a dispute over pricing and unpaid bills that marked the third such stoppage in a decade, after price rows in 2006 and 2009.
Previous "gas wars" have led to supply disruptions to Europe, which gets around a third of its gas from Russia, and 40 percent of this via Ukraine.
Gas supplies to Europe have been unaffected by the latest row, but Ukraine's chaotic finances have left it struggling to keep up with regular pre-payment for its gas from Russia's state-controlled producer Gazprom.
"(There is) pre-paid gas for exactly two days, no action has been taken yet," Putin told a governmental meeting on Wednesday.
"I would ask the prime minister and Gazprom to pay more attention to this, given that no one needs conflicts similar to those in previous years. We are ready to strictly fulfil (our) contractual obligations, but under pre-payment only."
The European Commission helped to negotiate a "winter gas deal" between Moscow and Kiev, under which Ukraine paid off some of its gas debts and imported over 1 billion cubic meters of gas. The deal expires at the end of March.
Russia and Ukraine have agreed to discuss a summer package later this month, with Kiev winning an assurance at EU-mediated talks this week that it would not have to pay for energy delivered to rebel-held areas.
With economic output falling and a pro-Russian rebellion simmering in its eastern industrial heartland, Ukraine's gas consumption is likely to fall to some 40 billion cubic meters (bcm) this year from 42.5 bcm in 2014, the UNIAN news agency quoted the energy minister as saying.