Gazprom warned Ukraine on Tuesday it owed the Russian natural gas giant nearly a billion dollars in overdue back payments, amid moves by Kiev to strike a partnership agreement with the EU.
The state-run Russian firm's chief executive Alexei Miller said Gazprom had already extended the deadline on $882 million (640 million euros) in payments owed by Ukraine's government-owned gas company until October 1.
"But now October is ending and the bills are still not paid," Russian news agencies quoted Miller as saying.
"We are very concerned by the situation," said Miller.
"The situation is very serious. ... This issue must be resolved -- and it must be resolved now."
Moscow has been frequently accused of using its gas exporting monopoly as a diplomatic weapon aimed at influencing decisions of ex-Soviet nations that rely on Russian gas and contemplate closer relations with the West.
Gazprom has already twice interrupted shipments to Ukraine -- once in January 2006 and then again in January 2009 -- in moves that also disrupted supplied to some central and western European countries at the height of winter heating seasons.
Ukraine hopes to strike an association and trade agreement with the European Union at a November 28-29 summit in Vilnius, which would serve as the first step to Kiev's potential membership in the 28-nation bloc.
That deal remains uncertain because of EU leaders' demand on Kiev to release and pardon the jailed opposition leader Yulia Tymoshenko -- a rival of President Viktor Yanukovych who served as prime minister during Ukraine's 2009 gas dispute with Russia.
Moscow meanwhile wants to see Ukraine join a Russian-led customs union that already includes Belarus and Kazakhstan.
Russian President Vladimir Putin's official spokesman said on Tuesday that Gazprom's warning was in no way related to Ukraine's hopes of striking the EU pact.
"Without a question, the gas debts are not a political issue and are in no way related to the subject of the associated membership (agreement) with the EU," ITAR-TASS quoted Kremlin spokesman Dmitry Peskov as saying.
Ukrainian producers have already suffered in recent months from a Russian ban on some of its chocolates and the introduction of strict new customs checks.