Iran’s agreement to allow a United Nations inspection team to visit its secret nuclear facility in Qom shows, as the White House remarked guardedly, that it is moving “in the right direction”. There is a very long way still to go. This concession, wrung from Iran’s nuclear negotiator at last week’s confrontation with the five Security Council permanent members plus Germany, was a desperate attempt to play for time. Iran had been caught red-handed by Western intelligence, which revealed the Qom plant at a moment calculated to cause maximum embarrassment. Unless Tehran moved swiftly, it faced exposure at the UN General Assembly as a serial liar, fresh UN sanctions and the very real threat of an Israeli military strike.

So the tone taken in Geneva last Thursday by Saeed Jalili, Iran’s negotiator, was in striking contrast to the bluster of President Ahmadinejad, who only three weeks ago said Iran would never abandon its nuclear programme to appease Western critics. Mr Jalili refused to accept the West’s long-standing offer of a freeze on nuclear enrichment in return for a freeze on new sanctions. But he did make two concessions unthinkable a month ago: he agreed to an immediate visit by Mohamed ElBaradei, the head of the International Atomic Energy Agency (IAEA); and he appeared to accept an idea first floated by Russia some years ago that Iran should ship 1,200kg of low-enriched uranium to either Russia or France to be enriched for use as fuel rods for Iran’s research reactor.

Mr ElBaradei has now made his visit, and yesterday arranged to send his IAEA team to Qom on October 25. He, too, appears to have been caught out by this secret plant: only a month ago he said that the threat that Iran was developing a nuclear weapon was exaggerated. In the past two years he has repeatedly played down Western concerns, but even he could not explain away the Qom facility. He reproved Iran on Saturday for not declaring it earlier. But he still insisted there was no “concrete proof” that Iran was developing nuclear weapons.

However, according to leaked excerpts of an analysis contained in an IAEA report, nuclear weapons experts say that there is information to suggest that Iran has the know-how to make an atomic bomb and has worked on the military technology to produce and deliver such a weapon. This goes further even than the Pentagon’s assessment, and vindicates the scepticism of those who doubted an earlier US intelligence report that work on a weapon had stopped. The leak from the IAEA may have been an attempt to force the hand of its departing director: in any case, it lends the visit to Qom critical importance.

This latest exposure of Iran’s true intentions underlines two vital facts that must guide Western policy. First, Iran will lie and lie again about its nuclear programme, admitting details only when caught. That makes the delay in the visit to Qom worrying: there is plenty of time to dismantle the laboratories, conceal the research and present a false picture of what is going on there. Second, realpolitik is the final determinant of Iranian policy. Russia, pleased by the abandonment of the US anti-missile shield and angered by the Qom revelation, is unlikely to veto new Security Council sanctions, and China is unlikely to hold out on its own. The West must press home its advantage. Only by holding Iran’s feet to the fire can its nuclear programme be halted or slowed.