"Iran is feeling the bite of these sanctions in a substantial way. The world is unified, Iran is politically isolated. And what I have said is that we will not countenance Iran getting a nuclear weapon," Obama told reporters at a White House press conference.

"We're now seeing noises about them returning to the negotiating table, that it is deeply in everybody's interests, the United States', Israel's, and the world's, to see if this can be resolved in a peaceful fashion.

"And so this notion that somehow we have a choice to make in the next week or two weeks or month or two months is not borne out by the facts."

Israeli Prime Minister Benjamin Netanyahu warned during a US visit on Monday that it could not afford to wait "much longer" for sanctions to work, and said he would "never let my people live in the shadow of annihilation".

Obama lashed out at criticism from Republican rivals over his Iran policy, saying "bluster" is not helping resolve the crisis and accusing them of repeating the same policies he has been pursuing for three years.

"This is not a game, and there's nothing casual about it," the president said. "When I see the casualness with which some of these folks talk about war, I'm reminded of the costs involved in war."

Republican frontrunner Mitt Romney suggested he would be more willing than Obama to consider using military force while his main rival for the presidential nomination Rick Santorum backed an ultimatum demanding Iran stop nuclear production or face action by the US to "tear down" its facilities.

In Brussels, EU foreign policy chief Catherine Ashton, speaking on behalf of Britain, China, France, Germany, Russia and the United States, reiterated an offer to resume talks with Tehran.

The Obama administration has said it does not believe Iran has taken a decision to develop a nuclear weapon, or that the time is right for military action, preferring to give biting new sanctions time to work.

However Israel, which sees a possible Iranian nuclear weapon as a threat to its existence, believes that Iran may be on the cusp of "break out" capability -- the moment when it could quickly build a nuclear weapon.

Speaking to reporters a day after Israeli Prime Minister Benjamin Netanyahu met US President Barack Obama for top-level talks on the Iranian nuclear standoff, former military intelligence chief Amos Yadlin said the Jewish state was working to a much tighter deadline than Washington.

"Israel is very close to the point when a very tough decision should be made -- the bomb or the bombing," he said, adding that the decision was also in the hands of the US president.

Netanyahu and Obama met for two hours of talks on Monday amid clear signs of differences on the imminence of the perceived Iranian nuclear threat, if not its ultimate danger to both nations.

Israel sees an Iranian nuclear weapon as a threat to its existence, and believes Tehran may be on the cusp of "break out" capacity -- the moment when it could quickly produce weapons-grade uranium.

But Washington has indicated it does not believe Iran has yet taken the decision to build a weapon and that the time is not right for military action, although Obama did clearly acknowledge Israel's right to self defence.

"He didn't say it explicitly, but this can be seen, not as a green light, but as an amber light," Yadlin said. "If Israel sees it is in danger and it has to do something about Iran, he will understand."

With Iran shifting its core nuclear facilities into protected underground sites, Israel fears Tehran is moving into the so-called "immunity zone," where its infrastructure would be hidden far from the reach of an Israeli attack, he added.

But Washington, which has "much stronger and more capable airpower," was operating on a different timeline, with different objectives about when to move in with force, said Yadlin, who now heads Israel's Institute for National Security Studies.

"Israel is basically saying Iran should not be capable of deciding to break out for the bomb. The United States is saying: when they break out, we will stop them," he argued.

"So this gives you another year or two between the final time that Israel can do something, and the final time that America can do something to prevent Iran from being nuclear."

Yadlin described the strategy of the Islamic Republic as "sophisticated," saying it was unclear whether Iran had actually taken the decision to build a nuclear weapon.

A decision on going "the last mile" was likely to be taken "as late as possible," at a time when the world was busy with a different crisis.

Yadlin said it would be possible to find out if Tehran decided to produce weapons-grade uranium, and start enriching above 20 percent in secret.

"But what will be difficult at that point is to stop it," he warned.