A Russian energy official said the government was prepared to cut off natural gas deliveries to Ukraine should Kiev start "siphoning off" gas intended for European consumers from the transit pipelines.
Yuri Lipatov, chairman of the State Duma's energy committee, said the action would be taken for financial, rather than political reasons.
"If there is money, there will be gas. If there is no money, (all) gas will go to Europe. Should Ukraine become brazen again and start siphoning off transit gas for its own needs, the valve will be shut down," he warned.
Russian gas is one of the key issues in Ukraine as the nation gears up for presidential elections next January.
Yulia Tymoshenko, incumbent prime minister and presidential hopeful, on Monday rejected calls by rival politicians to privatise the nation's gas transit system.
"I shall never allow in my lifetime the privatisation of our gas transit system, creation of any international consortia, or other encroachments by foreign countries on our gas transportation system - it is our national wealth, that should for ever remain the state property of Ukraine."
Opposition politician Vladimir Ryzhkov claimed the dispute was motivated by politics, not money.
He said that, since there had been no disputes between the two countries before the so-called Orange Revolution in 2004, which saw Viktor Yushchenko confirmed as president, ahead of the Moscow-backed candidate Viktor Yanukovych, the dispute had to be political.
The Kremlin, he said, wanted to exert pressure on Ukraine and affect the results of the presidential elections.
And he called on the European Union to play a part in trying to cool any disputes between the two sides.
Despite threatening to fine those countries that do not pay for their full amounts of contracted gas, Russia's Prime Minister Vladimir Putin has said Ukraine - which is in dire financial trouble - would not be fined.
Some observers have interpreted this as a gesture of support for Tymoshenko, who is running for president in January's vote.
Russia is the EU's largest energy supplier, providing 20 percent of the bloc's natural gas through the Ukrainian pipeline.
Putin has suggested the 27-nation EU help finance secure gas deliveries by lending $1 billion to Ukraine - an idea European leaders have dismissed.
On Monday, Russia and the EU agreed to create an early warning system to minimise damage from possible supply disruptions in the future.
APTN