Russian Prime Minister Vladimir Putin says Ukraine might be having problems paying for gas, raising new concerns over European supplies.
Mr Putin said the European Union has not yet given Ukraine the money it had promised to help provide stable supplies of Russian gas to Europe.
He also blamed Ukrainian President Viktor Yushchenko for blocking payment.
In January, many countries were left short of gas because of a payment dispute between Moscow and Kiev.
In August, the EU and international lending institutions agreed a loan deal worth $US1.7 billion (£1 billion at the time) to help secure European gas supplies.
This was specifically designed to ensure that disruptions to supplies that occurred earlier in the year, were not repeated.
The BBC reports Mr Putin's comments have raised concerns that disruptions may happen again.
Russia provides about a quarter of the gas consumed in the EU: 80% is piped through Ukraine.