The chief of Russia`s Gazprom said on Monday he is concerned Ukraine will not pay in full for this month`s gas supplies, Reuters reported.
"We see the payment situation regarding gas supplies in May as very, very grave," Alexei Miller said.
"If there is any disruption in payments for May supplies, Gazprom will have to move over to 100 percent advance payments."
Gas export monopoly Gazprom says it wants to store extra gas in Ukraine during winter to be able to respond more quickly to the needs of its customers in Europe.
How these storage facilities will be filled, and who will pay to fill them, have become the main sticking point in the most recent gas talks between the former Soviet allies.
A new spat over Ukraine`s proposal to defer payment of up to $5 billion in gas storage payments has also prevented agreements which could ensure smooth gas supply to Europe.
Russia, which supplies a quarter of Europe`s gas, much of it via Ukraine, has twice cut supplies in recent years due to pricing disputes between Moscow and Kiev, leaving parts of Europe with no gas in the dead of winter.
Russia has called on the EU to help Ukraine pay its gas bills and help avert a new gas crisis.
