BBC NEWS Urgent talks on Ukraine gas row Ukrainian President Viktor Yushchenko is in Moscow to try to persuade Russia not to cut gas supplies to his country in a dispute over an unpaid bill.
He has just five hours with President Vladimir Putin to settle the row over a demand by Russian gas monopoly Gazprom for Ukraine to pay $1.5bn.
Russia is threatening to switch off supplies at 1500 GMT, but Kiev disputes the bill.
Gazprom said the disagreement would not affect supplies to the rest of Europe.
But as most of the company's gas supply to western Europe passes through Ukraine, concerns remain that the row could escalate into a repeat of the supply interruption that happened two years ago.
Deep suspicion
In January 2006, gas shipments to Europe were disrupted after Russia halted fuel supplies to Ukraine for several days amid a fierce argument over price hikes.
Although Gazprom claims the current dispute is purely commercial, fears have been raised that Moscow might be exploiting its energy resources to exert political pressure on its neighbours.
The BBC's Richard Galpin in Moscow says the crux of this latest crisis seems to be over how the bill is to be paid.
Ukraine's pro-Western Prime Minister Yulia Tymoshenko wants the money paid directly to Gazprom, rather than going through an intermediary company - RosUkrEnergo - which she views with deep suspicion.
On Monday, Gazprom and the Ukrainian state energy companies failed to resolve the gas debt dispute during several hours of talks.
Gazprom extended its deadline for cutting a quarter of gas supplies to Ukraine until 1500 GMT on Tuesday - raising hopes the row could be settled in time.
Ukraine's prime minister has suggested that Gazprom's threats are empty.
She said Russian First Deputy Prime Minister Sergei Ivanov had informed her supplies would not be cut.
Our correspondent says Moscow does not like President Yushchenko or his prime minister.
Both were leaders of the Orange Revolution three years ago that swept away a pro-Russian government in Ukraine and replaced it with one that looks to the West for support and now wants to join Nato.
US state department spokesman Sean McCormack said Washington was monitoring the gas dispute.
In the Ukrainian capital, Kiev, people are preparing for another interruption of the service.
"Of course we are very bothered by the gas problem," one resident told the Associated Press news agency.
"It already became a tradition in Ukraine - every year they threaten to shut down our gas. Is it normal? It looks like our government is not able to make a deal with somebody there [in Moscow].
Another said it was time "to find a compromise and understanding with Russia as well as with the European Union. We are all people who deserve respect, the right to be listened to and the right to be understood".
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