Israel is "very close" to making a tough decision on whether to attack Iran's nuclear facilities while it still has the chance, a former Israeli intelligence chief said on Tuesday.

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.