Yesterday's letter from Mahmoud Ahmadinejad to George Bush, the first such publicly announced communication since the 1979 Iranian revolution, may represent a tentative but significant first step towards direct talks with the US. Given the high level of international concern over Iran's nuclear activities and the possible American response to them, that could make it one of the more important missives to land on the White House doormat in recent years.

But it is equally possible that Iran, facing growing diplomatic isolation, is attempting to exploit divisions within the US and the UN security council over the nuclear issue. Mr Ahmadinejad's vague but tantalising offer to seek "new solutions" to international problems may stiffen Russian and Chinese opposition to the potentially fateful UN resolution the US, Britain and France hope to pass this week.

The Iranians will certainly have noted the lengthening procession of senior Republicans urging the Bush administration to swallow its prejudices, stop working through European proxies, and talk directly to Tehran. John McCain, Mr Bush's rival for the Republican nomination in 2000 and a likely presidential candidate in 2008, added his voice at the weekend. Direct talks were "an option you probably have to consider", he said.

Richard Lugar, the Republican chairman of the Senate foreign relations committee, has proposed that Iran join a dialogue on energy and security with India, China, the US and other countries. It was too soon to press for UN sanctions, he told ABC television. "I believe for the moment we ought to cool this one." Despite accusations of appeasement from neo-cons, pressure on the US to talk rather than threaten has also come from Richard Armitage, the deputy secretary of state in Mr Bush's first term, Kofi Annan, the UN secretary-general, and Mohamed ElBaradei, the UN nuclear chief.

Mr Bush has ignored such calls so far, relying in part on the dangled threat of military action to influence Tehran. Limited US talks with Iran on Iraq have yet to begin. But impatience tinged with alarm over the dangers inherent in the impasse is also building in western Europe.

"There is a growing feeling on the European side that the US needs to get involved and broaden the agenda," one diplomat said, adding that the Americans were being privately pressed to change tack. Germany, where opposition to US military action is strongest, has publicly urged talks.

Mr Bush's formal response to Mr Ahmadinejad's letter will be crucial. The temptation will be to view it as a sign of weakness and rebuff it, as happened in May 2003 when Tehran proposed unconditional, comprehensive talks. That offer, conveyed as now via Swiss diplomats, received no response from a White House buoyed by the fall of Baghdad. But Mr Bush was far stronger, politically and militarily, then than now. A generation of enmity, sharp ideological differences, and personal distaste will be weighed against pragmatic considerations. Flat rejection will be interpreted by Tehran and others as confirmation that the US is using the nuclear issue as a lever for regime change.

Asked whether the letter signalled a policy shift, a Tehran source said: "When you write a letter to someone, you expect to get a reply. Maybe it's good news." But the situation in Tehran was "extremely political, extremely sensitive". Very little was required to tip it back into confrontation.