Leading article: A letter that keeps diplomacy alive

The President of Iran has written a letter to the President of the United States. It is the first "summit" level communication between the two countries since Washington broke off relations over the 1979 hostage crisis. Whatever the letter says - and last night we had only the barest summary from the Iranians - its very existence is a statement of sorts.

So is the fact that the Iranians want the outside world to know about it. Until now, President Mahmoud Ahmadinejad has preferred fiery rhetoric tailored to the aspirations of his home audience. The dispatch of a letter is the latest and most serious hint that Tehran might be a little more flexible than the incendiary flights of oratory would suggest. Even if it restates Iran's known position, a letter reflects a desire to communicate. It would be wrong to dismiss it out of hand.

Washington has not done so - yet. The US intelligence supremo, John Negroponte, hypothesised that Tehran was trying to influence the coming UN Security Council debate on its nuclear programme. Well, you do not need to be head of US intelligence to figure that out. A more hopeful interpretation might be that Iran has realised the gravity of its situation and wants to get its side of the argument heard.

Contrary to the impression it often gives, Tehran has not been immune to pressure in recent months. But it was never clear whether the conciliatory noises had the support of Mr Ahmadinejad or the ayatollahs. Latterly, Iran could also retort that the US, too, seemed to speak with a forked tongue. On the one hand, Washington said fiercely that there was nothing to discuss with Iran; on the other, the US ambassador in Baghdad was authorised to open up a channel of communication - though not, supposedly, to discuss the nuclear issue.

Nor are these the only fluid elements in the current situation. As the draft of a US-European resolution that could lead to sanctions goes to the Security Council, Russia and China have made known their disapproval. While Turkey seems to have been roped in as go-between, Iran is also mobilising support elsewhere in the Muslim world. Yesterday, on the eve of a visit by the Iranian President, Indonesia publicly supported Iran's right to develop nuclear technology for peaceful purposes.

Iran has some valuable cards in its hand, and it knows this. Among them are its oil and its threat, repeated at the weekend, to withdraw from the Nuclear Non-Proliferation Treaty. Diplomacy may be slow and frustrating for all concerned, but so long as the avenues of communication are still open - as they clearly are - it should not be abandoned. The alternatives could be counterproductive, or worse.