Long denied access to foreign technology because of sanctions, Iran has nevertheless learned how to make virtually every bolt and switch in a nuclear weapon, according to assessments by U.N. nuclear officials in internal documents, as well as Western and Middle Eastern intelligence analysts and weapons experts.

Iran's growing technical prowess has been highlighted by a secret memo, leaked to a British newspaper over the weekend, that purportedly shows Iranian scientists conducting tests on a neutron initiator, one of the final technical hurdles in making a nuclear warhead, weapons analysts said Monday.

There was no way to establish the authenticity or original source of the document, which is being assessed by officials at Western intelligence agencies and the U.N. nuclear watchdog.

Even so, former intelligence officials and arms-control experts said that if it is a genuine Iranian government document, it is a worrisome indication of an ongoing, clandestine Iranian effort to acquire nuclear weapons capability. Iran has steadfastly denied seeking nuclear arms.

The accumulating evidence of Iran's nuclear momentum emerges as Secretary of State Hillary Rodham Clinton conceded Monday that the White House has little to show for nearly a year of diplomatic engagement with Iran over its nuclear ambitions. "I don't think anyone can doubt that our outreach has produced very little in terms of any kind of a positive response from the Iranians," Clinton told reporters.

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