President Bush is to hold White House talks with the Israeli Prime Minister Ehud Olmert on Monday after publication of a nuclear watchdog’s report this week showing that Iran may have stockpiled enough enriched uranium to make a nuclear bomb.

The International Atomic Energy Agency believes that Iran has amassed 630kg of low enriched uranium, up from 480kg in late August. Some experts believe this is enough to produce the weapons-grade material needed for a crude nuclear device similar in size to that which America used to destroy the city of Nagasaki at the end of the Second World War.

Sean McCormack, the US State Department spokesman, said: “It’s concerning. This is a matter that will be taken up next week at the IAEA Board of Governors meeting.” Asked if Tehran now had sufficient material to build a bomb, he suggested that there were different opinions. “Some said it was enough; others said it was not enough, but close,” said Mr McCormack. “In any case, you don’t want Iran to get close.”

In its report, the IAEA said that Iran was working hard roughly to double its number of operating centrifuges. European diplomats say that Iran might have 6,000 centrifuges enriching uranium by the end of the year - and plans to install another 3,000 early next year.

The White House, announcing the meeting with Mr Olmert, gave little indication that Tehran’s nuclear ambitions would be at the top of the agenda. Mr Bush’s press secretary, Dana Perino, said that they would discuss “the continuing efforts to bring peace to the Middle East and a wide range of international issues”.

The timing of the talks, between two lame duck leaders with only weeks to go before they leave office, is intriguing. Israel has stated repeatedly that it would be unacceptable for an Iranian regime to acquire nuclear weapons. Altough Tehran insists that its uranium enrichment programme is for peaceful purposes, President Ahmadinejad has vowed to wipe Israel off the face of the earth.

During his visit to Washington, which begins on Sunday, Mr Olmert is expected to meet the Vice-President Dick Cheney, the Secretary of State Condoleezza Rice, and Robert Gates, the Defence Secretary.

Intelligence sources have told The Times that the prospect of Israel taking preemptive military action to knock out Iran’s nuclear facilities appears to have become significantly more likely in recent weeks. Such an operation would require at least tacit US cooperation because it would almost certainly involve Israeli warplanes flying through US-controlled airspace in Iraq.

Barack Obama is known to have raised this issue when he visited Israel last summer. Diplomats in Washington have confirmed that he discussed with European leaders concerns that the first months of his presidency could be thrown off-course by such a military crisis.

Although he campaigned on a promise to talk to America’s enemies, Mr Obama hopes a more moderate government will emerge from Iran’s own elections in June before he opens negotiations with Tehran.

European leaders are in favour of greater engagement with Iran, but wary of undermining the tough united front adopted with the US. The UN Security Council has made four resolutions in two years requesting Iran to halt its uranium enrichment activities.

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