Perry told a foreign policy forum here that Obama must find a new diplomatic approach to stop Iran's suspected nuclear arms quest because Israel -- which has held out the threat of military action -- will not "sit idly by."

The star-studded event sponsored by the United States Institute of Peace (USIP) was dominated by calls for Obama to check the spread of nuclear know-how and to promote peace in security in Iran, Iraq, Afghanistan and Pakistan.

"If Iran and North Korea cannot be contained, we are facing a real danger of a cascade of nuclear proliferation," Perry warned said. "Indeed I believe that today we are truly on the tipping point of nuclear proliferation."

Perry said North Korea's production of plutonium and nuclear test in 2006 amounted to the "most dangerous development" since the Cold War ended nearly two decades ago, but he believed it could be contained through diplomacy.

In multilateral talks pursued by President George W. Bush's administration, North Korea agreed in 2007 to dismantle its weapons-grade plutonium program in exchange for aid, although the talks are stalled amid a row over verification.

But Perry was "less confident" about diplomatic efforts involving the United States -- under the outgoing administration of President George W. Bush -- Britain, France, Germany, Russia and China.

The multilateral talks "are going nowhere," said Perry, who was defense secretary under President Bill Clinton.

"And it seems clear that Israel will not sit by idly while Iran takes defiant steps toward becoming a nuclear power," Perry said.

"As a result President Obama will almost certainly face a serious crisis with Iran. Indeed, I believe that crisis point will be reached in his first year in office," he added.

He said the Iran problem needs international cooperation, which remains especially elusive as US relations with Russia are "at an all-time low" with Russian concerns over NATO expansion and US missile defense plans.

However, he voiced hope that Obama could get off to a fresh start with Russia.

Perry said efforts to rid the world of nuclear weapons that began after the Cold War have "stalled and even reversed," because of developments in Iran and North Korea, as well as in China, Russia, India and Pakistan.

He supports Obama's stated push to abolish nuclear weapons, but said the United States needs to bolster its deterrent force over the short term.

"I'm motivated ... by a strong belief that the gravest danger facing our nation today is a terror group detonating a nuclear bomb in one of our cities," Perry said.

James Schlesinger, another former US defense secretary, told a separate audience that Iran may regard the threat of a US nuclear attack as "much more likely" in light of Hillary Clinton's warning during the US presidential campaign that Washington can obliterate Tehran.

Schlesinger made the remark at a Pentagon news conference after presenting a blue-ribbon panel report finding that US nuclear deterrence has slipped due to neglect in past years at high levels of the Pentagon.

Schlesinger, who served under presidents Richard Nixon and Gerald Ford, said North Korea probably has come to believe it is "reasonably safe from a nuclear response" because of the US response to its development of nuclear weapons.

But asked whether Iran feared a US nuclear attack, Schlesinger said: "I think they would regard that as a much more likely development."

"As you may recall in the recent democratic primaries, Mrs Clinton observed, 'We can obliterate you','" he said,

Noting that Obama has chosen Clinton to be his secretary of state, he added "I don't think that remark will be forgotten in Tehran, even if it is forgotten in this country."

At the USIP forum, Zbigniew Brzezinksi, the national security adviser under president Jimmy Carter, warned against any US war with Iran because it would inflame an already troubled region from Iraq to Afghanistan to Pakistan.

"It is an issue which I would prefer not to confront, for such a war would involve the United States in a conflict that would then spanning in different degrees of intensity these four countries with a total population of approximately 300 million people," he said.

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