A new gas war between Russia and Ukraine threatened to hit supplies to Europe yesterday.

Ukraine's Naftogaz said that it could siphon off gas destined for western Europe after Russia's Gazprom announced that it would halve deliveries in a dispute over unpaid debts and a new delivery contract.

Naftogaz said that it would "defend the interests of Ukrainian consumers". Asked if this included taking gas intended for Europe, spokesman Valentyn Zemlyansky said: "We don't rule that out — Europe must also understand to what extent Ukraine can be bent."

Russia supplies a quarter of Europe's gas, much of it through pipelines that cross Ukrain. Supplies were hit in January 2006 when state-controlled Gazprom turned off the taps in a row with Ukraine over price increases.

It has promised that the dispute with Ukraine this time will not affect its European customers. Gazprom cut supplies to Ukraine by 25 per cent on Monday and said it would halve them from Tuesday evening unless Naftogaz settled the dispute.

Gazprom threatened to cut supplies last month after saying that it was owed $1.5 billion by Ukraine. That warning was timed to coincide with a visit by Ukrainian President Viktor Yushchenko to Moscow for talks with President Vladimir Putin. The two men struck a deal to settle the issue but Naftogaz has not signed documents confirming the agreement.

Naftogaz accused Gazprom of waging "unprecedented psychological pressure" against it over the contract to supply a mix of Russian and cheaper Central Asian gas. It said that the dispute centred on the use of an intermediary company to sell gas to Ukraine.

The dispute with Russia is threatening to tear apart the fragile coalition between Mr Yushchenko and his former Orange Revolution ally, Prime Minister Yulia Tymoshenko. Ms Tymoshenk has refused to accept the agreement struck in Moscow despite a warning from Mr Yushchenko not to spark a "gas war" with Russia.

"They are like two spouses whose marriage of convenience has been transformed into mutual hatred," said Volodymyr Fesenko, a political analyst at the Penta research centre in Kiev.

In a sign of the tensions, Foreign Minister Volodymyr Ohryzko, a key ally of Mr Yushchenko, insisted that supplies to Europe would be unaffected by the dispute.

"I assure you that Ukraine will do everything so that supplies to the European Union continue as usual," he told reporters in Warsaw, Poland.

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