President Bush is prepared to bypass the United Nations and instead work with European allies on imposing tougher sanctions against Iran’s defiant stance on its nuclear programme.

He is understood to be increasingly frustrated by the snail-pace progress of four months-long talks with Russia and China over punishing Tehran for failing to comply with successive Security Council resolutions. The west fears that Iran’s avowed determination to secure nuclear energy is cover for pursuit of a military programme.

But, even as officials from the US, Russia, China, France Britain and Germany met in Washington today to consider the next steps, diplomats admitted that America and the three European powers might be forced to “go it alone”.

Such a decision would represent a significant crack in the united front the world has so far presented to Iran. It would also be seen as evidence of the tensions within the Bush administration over how far the US will follow the diplomatic route rather take a military option - which hawks have determinedly kept “on the table”.

Although the US already employs a full range of economic sanctions against Iran, Washington has repeatedly pressed European banks and energy companies to cease investing in a state it lists as a state sponsor of terrorism. Firms that could be hard hit include the Anglo-Dutch oil giant, Shell, which is considering a multi-billion pound project in Iran to produce natural gas.

The new French government of Nicolas Sarkozy has been particularly receptive to calls for further sanctions in recent weeks. A spokesman for the French president said agreement on further UN sanctions “could take time for this reason we are thinking of additional measures”.

Britain and - to a lesser extent - Germany, are also sympathetic, but remain more marginally optimistic than the US about securing progress when ministers to the UN General Assembly next week.

Mr Bush insisted on Thursday that “the objective, of course, is to solve this peacefully,” adding: “It’s imperative that we continue to work in a multilateral fashion...and one place to do so is at the United Nations.” But Russia and China have made clear they are opposed to additional UN sanctions that Iran has promised to answer unresolved questions about its past nuclear programmes by the end of the year.

Vitaliy Churkin, Russia’s UN ambassador, said today: “One can impose sanctions almost every day but this won’t bring the resolution of the problem closer.” But US officials point out that by the end of this year Iran could have installed many more centrifuges at its Natanz plant used for uranium enrichment.

Condoleezza Rice, the US Secretary of state, said today: “A number of companies are not going to deal with Iran. And I think it is extremely important that governments signal .that it is not business as usual.” Speculation that the US could be planning airstrikes to destroy the uranium enrichment plant at Natanz has been heightened by Israel’s bombing on September 6 of a suspected nuclear site in Syria.

The Washington Post reported today that Israel’s decision to attack as based on intelligence shared with the US that North Korea may have been assisting Syria - a country linked with Iran - fulfil its nuclear ambitions.

Details of the operation remain shrouded in deep secrecy. Mr Bush’s refusal on it to comment this week has been attributed to his concern about undermining long-running negotiations designed to persuade North Korea to scrap its own nuclear programme.

But the mystery may also reflect acute sensitivity among Washington’s neo-conservatives about the wisdom of pursuing diplomatic solutions with states such as North Korea and Iran - both of which were once branded part of the “axis of evil” by Mr Bush.

British officials, however, insist that persistent reports about US military plans for an airstrike against Iran are being cooked up “by Sunday newspaper journalists who can get retired generals and think tank people to say whatever they want”.

Sir David Manning, Britain’s ambassador to Washington, told The Times that he has detected no sign from the US Administration “at the moment” that they are “near to abandoning the diplomatic route”.

Mahmoud Ahmadinejad, the Iranian president who is flying to New York tomorrow for the UN meeting, has already been stirring trouble by threatening to pay a visit to the site of the 9/11 attacks on the World Trade Centre.

Today he expressed amazement at a decision by New York police to refuse him access to the site. One official commented: “This is typical of the Iranians, they will do everything they can to grab the headlines and split us all up.”

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