Russia's state-run gas monopoly, Gazprom, is warning that it will cut natural gas supplies to neighboring Belarus by 45 percent beginning Friday, because Minsk has not paid for previous shipments.
Gazprom says Belarus missed a July 23 deadline for paying nearly $500 million for Russian gas consumed since January.
The firm said in a written statement Wednesday that it will do all it can to maintain natural gas supplies through Belarus to other European customers.
Belarus has been seeking to borrow money from Moscow to repay the half-billion-dollar debt and to stabilize its economy. But negotiators have failed to reach agreement on the loan in Moscow talks that began last week.
Russia pumps about 20 percent of its gas to central and western Europe through Belarus. The other 80 percent goes through Ukraine.
The French news agency, AFP, quotes a European Commission spokesman, Martin Selmayr, as saying the EU is concerned about the gas dispute and hopes it will be resolved quickly.
In January, Minsk imposed a transit duty of $45 per ton of Russian oil pumped to Europe, after Russia more than doubled the price it charges Belarus for natural gas.
The stand-off briefly interrupted the flow of Russian oil to Europe, before Belarus rescinded the transit duty and reluctantly agreed to the Russian gas price increase.
Moscow came under strong international criticism for the brief cut-off, with Western governments questioning Russia's reliability as an energy supplier.
Some information for this report was provided by AFP, AP and Reuters.