But he quickly qualified himself, adding that all nations should be concerned about whether Iran could make nuclear weapons.

"Clearly, if I was the leader of Israel and I'd listened to some of the statements by the Iranian ayatollahs that regarded the security of my country, I'd be concerned about Iran having a nuclear weapon as well. And in that Israel is our ally, and in that we've made a very strong commitment to support Israel, we will support Israel if her security is threatened."

His comments appeared to be a departure from the administration's line that there are no plans to attack at present and that Washington backs European diplomatic efforts. The remarks may have reflected Mr Bush's personal thinking on an issue causing deep concern in Washington.

Moments later, Mr Bush was asked another question on Iran and appeared to return to his script - this time emphasising the need for a diplomatic effort.

Speaking days before he arrives in Europe on a tour designed to mend fences with estranged allies, he underscored the differences still hobbling western policy towards the Middle East.

Many figures close to the United States administration believe that the European diplomatic initiative is calculated more to dilute America's hardline approach to weapons of mass destruction than to stop Iran's mullahs building a bomb.

Israel, meanwhile, has given warning about Iran's nuclear ambitions, saying that an Iranian bomb might be only six months away and that such a weapon would pose a grave risk to its security. Mr Bush repeated the reasons for America's anxiety: "Remember, this all started when we found [Iran] enriching uranium in an undeclared fashion, and it happened because somebody told on them."

Iran's long march towards becoming a nuclear power appeared to make a significant step forward yesterday with the opening of a £450 million reactor at Bushehr.

A senior Russian nuclear official said he would go to Iran next week to sign a protocol agreeing the return of spent nuclear fuel, the last remaining obstacle to Bushehr's functioning. This will allow deliveries of Russian nuclear fuel.

The protocol's signing has been repeatedly delayed. It aims to ease concerns that Iran could reprocess spent nuclear fuel from Bushehr to extract plutonium, which could be used in nuclear weapons.

Iran's influential former president, Hashemi Rafsanjani, speaking yesterday after meeting the Syrian prime minister, Mohammad Naji al-Otari, said his country needed to create a powerful alliance with Syria, Iraq and other Arab countries.

Mr Rafsanjani, widely expected to run in Iran's June presidential elections, said the region must "stay completely vigilant vis-a-vis the US and Israeli plots".

America has this week stepped up its rhetoric against Syria following Monday's murder of the former Lebanese premier, Rafik al-Hariri. Pro-Syrian elements have been widely accused of involvement.

Mr Bush called on Syria yesterday to obey a UN resolution demanding that it remove its troops from Lebanon and restore the country's independence.

He said: "We've recalled our ambassador [from Damascus], which indicates the relationship is not moving forward; that Syria is out of step with the progress being made in the greater Middle East."