The sabre-rattling over Iran's nuclear progamme has grown louder as a defiant Tehran claimed to have conducted missile tests for a second day running, the US warned that it would defend its interests and its allies in the region, and Israel hinted it was ready to stage a preventive attack to destroy Iranian nuclear installations.

    With the latest tests - and the wide front-page coverage given to them by the national media - Tehran is signalling it will not be cowed by international pressure to end a programme which the West suspects is aimed at producing nuclear weapons, and that any attack by the US or Israel will be answered in kind.

    The tests, including launching the 1,250-mile range Shahab-3 missile that can hit Israel, should be "a lesson to our enemies", the commander of Iran's Revolutionary Guard was quoted as saying. But some of the talk may be bravado. Pentagon officials told CNN that surveillance suggested only a single missile was fired yesterday, apparently one that failed to launch on Wednesday.

    Even so, the show of strength drew an unprecedentedly blunt response from Washington and Israel. No one should doubt US resolve, said Condoleezza Rice, the Secretary of State, on a visit to Georgia. "We are sending a message to Iran that we will defend American interests and the interests of our allies."

    More ominously, Ehud Barak, the Israeli Defence Minister, noted pointedly that while diplomatic pressure remained the preferred way of persuading Iran to halt uranium enrichment, Israel "has proved in the past it is not afraid to take action when its vital security interests are at stake".

    Not by coincidence, the country also put on display one of its state-of-the-art Eitam spy aircraft, whose intelligence-gathering abilities would be vital in any co-ordinated assault on Iran's nuclear installations. This latest publicity only reinforces the message sent by Israel's recent military air exercises over the eastern Mediterranean, widely seen as a dress rehearsal for such an attack.

    Most analysts believe that for all bellicose talk, a pre-emptive attack, by the US at least, is most unlikely. "Everyone recognises what the consequences of a conflict would be," the Defence Secretary Robert Gates warned, among them possible closure of the oil lifeline through the Strait of Hormuz, the risk of generalised war in the Middle East and immense new strains on the fragile global economy.

    Pentagon commanders too do not want to plunge the country's overstretched armed forces into another war. An attack would be "extremely stressful" for US forces, Admiral Michael Mullen, chairman of the joint chiefs of staff and the country's top uniformed officer, warned a few days ago.

    But the jitters have been increased by the political calendar in Washington and Jerusalem. From a US perspective, if the Bush administration is to strike, it probably has to do so before the general election campaign moves into high gear this autumn. The possibility - many would say likelihood - that the next President will be the Democrat, Barack Obama, who favours negotiation with Iran, only heightens the urgency for anti-Iran hawks.

    In Jerusalem, a corruption scandal could bring down the Mr Omert's government in September. This is another reason for Israel, if it is determined to go ahead, to act sooner rather than later, even alone and without the explicit collaboration of the US.