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David Cameron today urged the US to bomb the Syrian regime, claiming there was a risk of a civilian “Armageddon” if the use of chemical weapons went unpunished.

The Prime Minister's appeal to Barack Obama to see through military action against the regime of Bashar Assad came as he firmly ruled out any British participation. President Obama also said more attacks could happen if nothing was done. Speaking in Sweden, he urged: "I do think we have to act because if we don't we are effectively saying that even though we may condemn it and issue resolutions... somebody who is not shamed by resolutions could continue with impunity."

He said the whole world had agreed a "red line" against the use of chemical weapons. "I didn't set a red line — the world set a red line," he said. Tensions in the Middle East soared as Russia dispatched a large missile cruiser to the Mediterranean in what was seen as a riposte to Israel and America test-firing a missile yesterday.

The cruiser, Moskva, bristling with weaponry, will be joined by a destroyer from Russia's Baltic fleet and a frigate from the Black Sea fleet, tasked with "protecting national interests".

It will arrive days before the US intends to start action to degrade Assad's regime. Earlier, Russian President Vladimir Putin warned America that any attacks would be "an aggression" unless endorsed by a United Nations resolution and warned that Russia had "plans" on how to respond to such a an event.

He warned: "President Obama hasn't been elected by the American people in order to be pleasant to Russia. And your humble servant hasn't been elected by the people of Russia to be pleasant to someone either."

In London, Mr Cameron sacked a member of his policy board for refusing to support the Government in last week's vote on military action. Conservative MP Jesse Norman was told to resign for abstaining on a three-line whip, even though Aid Secretary Justine Greening, who missed the vote by accident, is being allowed to stay on. Mr Cameron made more clear than ever that he will not take part in military action, telling MPs: "I accept that Britain can't be part, and won't be part, of any military action."

But he appealed to Mr Obama to stand firm on his "red line" that Syria's use of chemical weapons must be punished. "He set a very clear red line, that if there was large-scale chemical weapons use something had to happen," he said.

"To ask the President of the United States, having set that red line, having made that warning, to step away from it I think that would be a very perilous suggestion to make, because in response I think you would see more chemical weapons attacks from the regime."

To a claim by the Father of the House, Sir Peter Tapsell, that an illegal attack could cause Armageddon, Mr Cameron retorted: "You have to ask yourself what sort of Armageddon are the people of Syria going to be facing."

Speaking ahead of the G20 summit being hosted in St Petersburg tomorrow, Mr Putin insisted he did not rule out Russia agreeing to military action but made it unlikely by demanding clear proof that the Assad regime used banned chemical weapons.