Russian Prime Minister Vladimir Putin has said Ukraine might be having problems paying for gas, raising new concerns over European supplies.
Mr Putin said the European Union had 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.
"It appears we are again having problems with payments for our energy supplies, which is extremely regrettable. The EU has still not provided Ukraine with the money it has promised for that purpose," Mr Putin said.
"European leaders are referring us to the European Commission, while the European Commission leadership are evading discussing the issue with us."
But Mr Putin said that despite the lack of funding from the EU, Ukraine still had plenty of cash to pay its gas bills.
Referring to the country's large gold and foreign currency reserves, he said: "According to the Ukrainian Prime Minister [Yulia Tymoshenko], Yushchenko is obstructing normal co-operation between the central bank and the Ukrainian government and is blocking the transfer of relevant funds."
In August, the EU and international lending institutions agreed a $1.7bn (£1bn at the time) loan deal to help secure European gas supplies.
This was specifically designed to ensure that the disruptions to supplies that occurred earlier in the year were not repeated.
Mr Putin's comments have raised concerns that disruptions may happen again.
Russia provides about a quarter of the gas consumed in the EU and 80% of that is piped through Ukraine.