Russia has threatened to target the Ukraine with nuclear warheads if the former Soviet republic joins Nato and accepts the deployment of United States anti-missile defences on its territory.

President Vladimir Putin of Russia warned Ukraine's leader Viktor Yushchenko of "retaliatory actions" should his country join the Western alliance during a joint press conference in Moscow.

"It's frightening not just to talk about this, but even to think about, that in response to such deployment, the possibility of such deployments - and one can't theoretically exclude these deployments - that Russia will have to point its warheads at Ukrainian territory," he said.

The Russian and Ukrainian leaders had just held emergency talks in the Kremlin to avert a energy supply crisis over Kiev gas bill - a similar dispute two years ago led to power cuts across Europe.

Mr Yushchenko responded to the Russian pressure by insisting on Ukraine's right to decide its own foreign policy while stressing that his country's constitution would not allow US military bases on its territory.

"You understand well that everything that Ukraine does in this direction is not in any way directed at any third country, including Russia," he replied.

"We follow the principle that any nation has the right to define its own security. Our constitution does not allow deployment by a third country or bloc on Ukrainian territory."

Mr Putin has condemned Washington's plans to include Poland and the Czech Republic in a missile defence shield as a "new phase in the arms race".

Russia fears the shield will threaten its national security and tip strategic military balance in Europe.

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