TEL AVIV: The window for dialogue over Iran's nuclear programme is closing, the US ambassador to Israel said on Wednesday, insisting Washington holds no illusions over Tehran's agenda at the talks.

Speaking at Tel Aviv University a week after world powers held a second round of largely fruitless talks with Iran, abassador Dan Shapiro insisted Washington would not continue such dialogue forever.

“We don't intend on continuing talks for talks' sake. The window is closing,” he told delegates attending an international security conference at the university.

“We have no illusions that Iran may be using this to buy time,” he said.

“It is up to Iran to follow through. The burden falls on Iran to prove it is serious.” Iran and the P5+1 — Britain, China, France, Russia and the United States plus Germany — achieved little in two days of intense talks in Baghdad last week except for setting up another meeting in Moscow in June.

Shapiro said there were “significant differences” at the Baghdad talks, but also “a narrow common ground” which the parties would try to build on in the Russian capital. But the US envoy warned the “harshest sanctions” were yet to come. And he said the threat of military action against Iran's nuclear facilities was still open.

“As we apply all elements of American power to prevent a nuclear Iran, the United States takes no option off the table — that means a political component, a diplomatic component, an economic component and a military option,” he said.

Neither the United States nor Israel has ruled out the option of a military strike but the administration of President Barack Obama has made clear it favours diplomacy over force at this stage.

Israel has warned repeatedly that its window of opportunity for any pre-emptive military strike against Iranian nuclear facilities is closing rapidly as a result of Tehran's efforts to put the key ones into underground bunkers.

Speaking to delegates during the morning, Israeli Defence Minister Ehud Barak said it was crucial to act before Iran reached that point.

“It is impossible to sleep in peace when the Iranians are systematically moving towards the point after which Israel will not be able to do anything about it,” he said.

“You don't wait until they acquire the ability, until they build it, deploy it and prepare to activate that nuclear capability — and only then do you act. Because then it is too late, you cannot act, there is nothing left for you to do,” he said.

“The relevant moment in terms of policy is the last moment in which you can do something about the issue. Otherwise it becomes an issue for commentators and historians. And you have to pinpoint that moment responsibly.” Barak said Washington was working on a different timeline than Israel and had different objectives about when to use force.

“It's clear that there are differences in our attitudes, in the tempo of the ticking of the clocks,” he said. “It's no secret. Our clock is ticking faster.” But Shapiro said Washington would never allow Tehran to reach such a point.—AFP