Leading article: Accusations, denials and a weird form of diplomacy

The best that can be said of the latest exchanges between the United States and Iran is that a dialogue is being conducted - albeit of a bizarre variety. On Sunday, US officials presented what they said was proof that Iran was directly involved in supplying weapons to Shia militias in Iraq. Yesterday, the Iranian President, Mahmoud Ahmadinejad, responded in an interview with the US television channel ABC. He categorically denied the charges, accused the US of form in the fabrication of evidence, and said - by the by - that Tehran was prepared to talk.

These are perilous days in US-Iranian relations, as they are for Iraq and the region as a whole, with many different aspects simultaneously coming to a head. The new US commander in Iraq, Lt-Gen David Patraeus, has just taken over; his unenviable task is to impose security on Baghdad with the help of additional US forces. The UN deadline for Iran to suspend its uranium enrichment programme falls next week.

President Bush is under pressure at home from anti-war feeling and the new Democratic majority in Congress. Mr Ahmadinejad's position was weakened by electoral losses late last year. And the slaughter in Iraq continues: more than 76 people were killed in Baghdad yesterday in apparently co-ordinated bombings.

This is the context for the latest US allegations against Iran. It may, or may not, be relevant. Claims that Iran has been helping Shia militants in Iraq are not new. They were made 18 months ago by British diplomats in Iraq, who said Iranian-made devices were being used in the south. US officials have gone a step further. They produced parts of explosive devices they said originated in Iran, and they linked them to the Iranian Revolutionary Guard, and thence to the top Iranian leadership. They also implicated the five Iranians arrested in Arbil recently.

For all the care taken by the US to bolster its case - the weeks of delay in presenting it, the minute detail, the show of weapons parts - the presentation at the weekend was disturbingly reminiscent of the claims about Iraq's weapons of mass destruction that turned out not to exist. There was a similar lack of proof that the Iranian authorities were the direct suppliers and a similarly worrying insistence on anonymity for the briefers. If the "evidence" turns out to have been misleading, there will be no one identifiable to blame.

Even if the devices seem to be Iranian in design and manufacture, there are other plausible explanations, not least the close association between the Iranian and Iraqi Shias at grassroots level and the fact that many Shia militants were formerly exiled in Iran. It is also pertinent to ask why the US is pointing the finger at Iran and Iraq's Shias, when the insurgents doing most damage to US troops and the US-backed Iraqi government are not Shia, but the Sunnis who lost power with Saddam Hussein. Is the US administration using Iran as a scapegoat for its own failings in Iraq? Is it softening up international opinion for another show of military force?

Given the complaisance with which almost every part of the US establishment accepted the official line on Saddam's non-existent weapons, it is gratifying to observe that this time around senior Democrats in Congress have declined to take the administration at its word. They are treating the case against Iran with due scepticism, warning that resort to a military solution would be a grave mistake. Late yesterday, the international nuclear watchdog, the IAEA, the Germans (who hold the presidency of the EU) and the Iranians were all hinting at the possibilities for further diplomacy. So far, at least, this strange multilateral conversation has not been completely a dialogue of the deaf.