UN: Iran nuclear threat grows real

BRIAN WILLIAMS, anchor: There's also news tonight about Iran and the threat of nuclear weapons . There's a new UN report out. It says the threat has grown more real, and this happens to come amid reports Israel is again considering some sort of military strike . Our chief foreign affairs Andrea Mitchell has more.

ANDREA MITCHELL reporting: Even before the report's release, for days Iran has been denouncing it as a tool of the United States and Israel .

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MITCHELL: Iran 's President Ahmadinejad today called the head of the UN agent a puppet of the US. But the UN agency the IAEA reports new evidence that Iran is on the verge of a major nuclear breakthrough to know how to build a nuclear bomb . What is the proof? Experts say satellite images show Iran has built an enormous steel structure that could be used to test explosives used to trigger a weapon. A warhead small enough to fit on a missile that could hit Israel or Europe. Despite denials, experts say Iran had help from a former Soviet scientist with decades of weapons experience.

Mr. DAVID ALBRIGHT (Nuclear Expert): The evidence looks pretty clear that he -- the Iranians benefited tremendously from his expertise and used it to build a smaller nuclear weapons design. I think some will conclude from this that it is a smoking gun.

MITCHELL: For years, Israel and possibly the US have waged a cyberwar against Iran's nuclear program . They've slowed its centrifuges with a Stuxnet computer virus. And two Iranian nuclear scientists were mysteriously assassinated on their way to work. But experts say Iran has now overcome those hurdles and is back on track. Intelligence estimates differ widely on how close Iran could be to developing a bomb. Still, few military experts outside of Israel believe that military action could take out Iran's nuclear program , much of it buried deep underground.

General BARRY McCAFFREY, Retired (NBC News Military Analyst): They don't have the conventional air power , the air-to- air refueling to credibly strike the Iranian nuclear production facilities.

MITCHELL: Israel is already holding mock disaster drills in anticipation of an attack from Iran . The UN weapons report will only increase fears in a region already on edge. Andrea Mitchell , NBC News, Washington.