Russian gas monopoly Gazprom has warned Ukraine it will reduce its gas supplies from next Monday if a $1.5bn (£772m) gas debt is not paid off.
Gazprom says Ukraine's debt has built up since November 2007
Gazprom said only gas piped from Russia would be affected which would make up around 25% of Ukraine's total supply.
A dispute in January 2006 disrupted exports to several European countries.
Much of Europe is reliant on Russian gas piped through Ukraine but the EU says it has been assured by Gazprom its supplies will not be affected.
Gazprom spokesman Ilya Kochevrin told the BBC News website that the company had informed the European Commission about the dispute.
"We think we got strong support for our position and hopefully we can settle this very soon," he said.
EU Energy Commissioner Andris Piebalgs called on both sides to find a solution as soon as possible.
On Wednesday, Ukraine joined the World Trade Organisation after 14 years of negotiations. Russia is the world's only major economy not to be a member.
Ukraine's gas imports were already high on the agenda for a summit in Moscow next week involving Russian President Vladimir Putin and Ukrainian President Viktor Yuschenko.
But Mr Kochevrin insisted the dispute was not political in any way.
"We're a commercial company: our investors won't understand if our profits fall," he said.
"We're accused again and again of political bias. I can assure you there is no political bias. Our link to the Kremlin is only through our management board."
Gazprom says the $1.5bn debt is made up of $1bn accrued from Central Asian exports at the end of 2007 and a further $500m of Russian gas piped to Ukraine in January 2008.