Russian President Dmitry Medvedev has warned Belarus that unless it pays off its debts for Russian gas within five days it will face cuts.
Russia's gas supplier, Gazprom, says Belarus owes about $200m (£166m), after failing to pay increased prices.
"We will give our Belarusian colleagues five days to decide how to handle this," Mr Medvedev told Gazprom chief executive Alexei Miller on Tuesday.
Otherwise, he told Mr Miller, Russia would resort to "other measures".
In televised comments, Mr Miller had just outlined how Gazprom could cut supplies in proportion to the debt.
Belarusian President Alexander Lukashenko visited Moscow on Friday and complained of financial difficulties, Mr Medvedev said.
But the Russian president said Belarus was not alone. "We have our own problems and Gazprom has many problems too," he said.
Russia and Belarus are supposed to be close allies but have had several rows in recent years, particularly over energy supplies.
Long-standing plans for a customs union, meant to be introduced this year, are still in doubt.
Russia increased the price of gas supplied to Belarus from $150 per 1,000 cubic metres of gas last year, to $169.20 in the first quarter of 2010 and $184.80 in the second.
But Belarus has continued to pay at $150.
Gazprom said at this rate it could owe $500m or $600m by the end of the year.
Russia has not been afraid to cut supplies to countries it accuses of falling behind in their payments.
In January 2006 and again in 2009, Russia cut gas supplies to Ukraine, causing knock-on effects all over Europe.
In January this year a row nearly resulted in Russian oil deliveries to Belarus being halted.
Russia's critics have accused it of using its energy supplies as a political weapon.