"I think the unintended consequence of a strike against Iran right now would be incredibly serious, as well as the unintended consequences of their achieving a weapon," Admiral Mike Mullen, the chairman of the Joint Chiefs of Staff said.

"That's why this engagement, dialogue is so important," he said in an interview on ABC television, referring to President Barack Obama's aim to engage Iran diplomatically.

Mullen said the United States would approach Iran "with all options on the table."

"So that would leave a pretty narrow space in which to achieve a successful dialogue and a succesful outcome, which from my perspective means they don't end up with nuclear weapons," he said.

Mullen said he did not believe Iran's claims that it is developing its nuclear capabilities for peaceful purposes, but he said the aim of diplomacy would be "to really bring out whether that is how the senior leaders feels."

"Certainly from what I've seen in recent years, Iran is on a path to develop nuclear weapons," he said.

"Most of us believe that it is one to three years (away from acquiring nuclear weapons), depending on assumptions about where they are right now. But they are moving closer clearly and they continue to do that," he said.

"And if you believe that is their strategic intent, as I do and certainly as my Israeli counterpart does, that's the principal concern," he said.

Israel's military intelligence chief asserted in March that Iran will have the capacity to build a nuclear bomb within a year, but was not rushing to produce one.

Obama told reporters after meeting with Israeli Prime Minister Benjamin Netanyahu in Washington May 19 that he expected to know by the end of the year whether the Iranian leadership were make "a good faith effort to resolve differences."

"We are not going to have talks forever," Obama said.

Mullen was asked in Sunday's interview whether it was possible to take out Iran's nuclear program militarily at an acceptable cost.

"I won't speculate on what we can and can't do," he said.

"Again, I put that in the category of my very strong preference is to not be put in a position where we -- where someone -- where Iran is struck in terms of taking out its nuclear capability," he said.

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