Cameron: Mistake to rule out military action against Iran

His comments came amid claims by Coalition officials that Iran is covertly working with rebel forces in Iraq in preparation for a major summer offensive to drive U.S. and British forces out of Iraq.

The Revolutionary Guard in Tehran has forged ties with al Qaeda and Sunni Arab militias in Iraq to wage war "by proxy" against the U.S., the Americans claim.

American commanders are now braced for a summer showdown aimed at persuading the US Congress into voting for a full military withdrawal.

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"Iran is fighting a proxy war in Iraq and it's a very dangerous course for them to be following," a senior US official told the Guardian. "They are already committing daily acts of war against U.S. and British forces."

"They (Iran) are behind a lot of high-profile attacks meant to undermine U.S. will and British will, such as the rocket attacks on Basra palace and the Green Zone," the official added.

He said the attacks were directed by the Revolutionary Guard, connected "right to the top" of the Iranian regime. He did not discuss how Shiite Iran had bridged its differences with Sunni forces in Iraq, where a sectarian civil war appears to be raging.

Conservative leader David Cameron called today for tougher international sanctions on Iran to bring pressure on Tehran to give up its nuclear weapon ambitions.

Mr Cameron said a "rapid" shift in policy was needed to beef up the current weak sanctions, and Iran must be made to realise that if it continues on its present path, it will become a "pariah state".

But he said Tehran must also be offered the carrot of possible talks with the USA if it complies with the demands of the international community.

Mr Cameron warned that the threat of a nuclear-armed Iran was "one of the most urgent" questions facing the world today.

Asked whether he would support military action against Iran, he said it would be a mistake to rule it out.

"We are trying to make sure that we do everything to avoid that happening. It would be a calamity if they got a nuclear weapon; military action would be calamitous."

Coalition commanders in Iraq say they have been finding "huge stockpiles" of Iranian weapons in recent months. They also warn of stronger links between Iran and al Qaeda, despite recent talks of al Qaeda attacks on Shiite targets.

One official in Baghdad-said he expected the violence to pick up before September when US commander General David Petraeus will report to Congress on the success or otherwise of President George Bush's six-month security "surge".

Mr Bush has sent 30,000 additional troops to the country in a final attempt to quell the insurgency. But the commanders on the ground fear that Iran-backed rebel forces are ready for a counter attack.

One official said the violence is "certain to pick up" ahead of the September deadline. "There is significant latent capability in Iraq, especially Iranian-sponsored capability. They can turn it up whenever they want," he said. "The whole Iran-al Qaeda link-up is very sinister."

Conservative MP Patrick Mercer, a former soldier, stressed the severity of the situation. "The message that keeps coming across is there's an undeclared war that's going on with Iran, both with personnel and weapons coming across the border," he said.

Gen Petraeus's report is being viewed as a pivotal moment for the military operation in Iraq. It will inform the decision on whether to continue-with the security "surge" or start the process of U.S. withdrawal. With the Democrats now dominant on Capitol Hill, Mr Bush may struggle to win approval for more military involvement. He will also be without his strongest international ally, Tony Blair.

The mood in Washington is that military action has failed in the face of the growing civil war in Iraq.