Clinton, Pentagon Reveal State Secret: Size of U.S. Nuclear Arsenal

The United States today for the first time since 1962 disclosed the size of its nuclear arsenal in what officials described as an as an unprecedented unveiling of a state secret and an attempt to encourage other countries to be open about their nuclear capabilities.

The Pentagon announced the U.S. currently has 5,113 nuclear warheads in its stockpile – an 84 percent reduction from a peak of 31,255 warheads in 1967.

"For those who doubt that the United States will do its part on disarmament, this is our record, these are our commitments, and they send a clear, unmistakable signal," said U.S. Secretary of State Hillary Clinton before the U.N.'s conference on the Nuclear Nonproliferation Treaty.

Her comments were a direct rebuke to those made earlier today by Iranian President Mahmoud Ahmadinejad, who said the U.S. has not met its non-proliferation commitments and uses nuclear weapons to threaten other countries.

"This morning, Iran's president offered the same tired, false and sometimes wild accusations against the United States and other parties at this conference," Clinton said. "That's not surprising… Iran will do whatever it can to divert attention away from its own record and to attempt to evade accountability."

The Iranian president denied in an address to the conference attendees that his country's nuclear program poses a threat to world security, saying that reports his country is producing nuclear weapons are based on "not a single credible proof."

He later prompted a walk out by delegations from the U.S., Great Britain and France, saying, "Regrettably the United States has not only used nuclear weapons but also continues to threaten to use such weapons against other countries including my country."

Last month, President Obama revealed the Nuclear Policy Review which vowed that the U.S. would not use nuclear weapons to attack any country. The policy included a caveat that said exceptions will be made for nations -- such as Iran -- that do not comply with the Nuclear Nonproliferation Treaty.

A senior Defense official said that with today's release of what is normally a closely guarded state secret, "the United States will set a standard for declassification of this information."

Iranian President Lashes Out at U.S.

During his speech, Ahmadinejad repeatedly depicted the U.S. as a rogue nation. In a series of shots, some veiled, at the U.S., Admadinejad said:

"Those who carried out the first atomic bombardments are among the most hated individuals in human history."

"The United States has never respected any of its commitments."

"It should be borne in mind that in the previous decade, the United States has made -- has had most of its wars and conflicts with those who were once its friends."

"Arming terrorists with nuclear weapons are only conceivable by those states which possess such weapons and have used them and also have a long record of supporting terrorists."

"Major terrorist networks are supported by the United States intelligence agencies and the Zionist regime. Credible evidence is available in this connection that will be publicized if needed during the forthcoming conference on the global fight against terrorism in Tehran."

The White House said Ahmadinejad's speech was full of "wild accusations."

Ahmadinedjad said every country should be willing to give up its nuclear arms and asked Obama this "humane movement" if he is still "committed to his motive of change."

Despite Ahmadinejad's insistence that there is no evidence of Iran working towards a nuclear weapon, U.N. Secretary General Ban Ki-moon called on Iran earlier in the conference to "clarify the doubts and concerns" that Iran is stockpiling and producing nuclear weapons.

The head of the International Atomic Energy Agency said at the U.N. he is "unable to confirm that all nuclear material is in peaceful activities because Iran has not provided the necessary cooperation."

"Iran's nuclear program is clouded, both because there are serious questions about the peaceful nature of that program and because they have not granted the IAEA the transparency necessary for them to address these questions," said Jacqueline Shire, a senior analyst and nuclear expert at the Institute for Science and International Security.

Ahmadinejad's visit comes at a sensitive time as the United States and its allies are pushing for another round of tough sanctions on Tehran.

The U.S. would like to see unanimous support on the United Nations Security Council for tough sanctions to isolate Iran, but Tehran has sought to drive a wedge between them and other countries on the council who are wary of supporting additional sanctions at this time, namely non-veto members Brazil and Turkey. Lebanon, whose government includes members of the Iranian-backed militant group Hezbollah, is not expected to vote in favor of a sanctions resolution. Lebanon holds the body's rotating presidency in May, making a vote on sanctions unlikely until June.

U.S. Hopes Release of Stockpile Data Will Advance Arms Control Talks

Both Brazil and Turkey have said they believe there is still time to negotiate with Iran. Turkey recently suggested it could help in a proposed deal that would send some of Iran's nuclear fuel abroad for enrichment before it is returned for the Tehran Research Reactor, thereby denying Iran the opportunity to use it in a bomb. That proposal was put on the table last October, but has yet to be accepted by Iran.

Brazil's president will visit Tehran later this month to try to persuade Ahmadinejad to negotiate. Brazil has been lobbied heavily by Secretary of State Hillary Clinton who traveled there in March to discuss the matter. She'll meet again today with Brazil's foreign minister.

The United States has modest goals for the NPT review conference, held every five years. U.S. officials say they hope most countries will agree to back strengthening the IAEA so that the nuclear inspections body can then step up its pressure on Iran.

"The Review Conference operates by consensus, which makes the adoption of bold, ground-breaking final documents unlikely," said ISIS' Shire.

Critical for the conference's success will be the support of Egypt, which leads the 188-members Non-Aligned Movement. Egypt, however, has said it wants to discuss one of the world's worst-kept secrets: Israel's undeclared nuclear weapons program.

The Obama administration has made nuclear security a cornerstone of its foreign policy. Last year Obama called for a nuclear-free world during a speech in Prague. This year, Obama traveled to the Czech capital again to sign a new nuclear arms reduction treaty with Russia. Last month Obama convened an unprecedented gathering of world leaders in Washington to discuss preventing the proliferation of nuclear material.

The U.S. hopes to avoid the failure of the last NPT review conference in 2005, which fell apart in large part due to concerns from many countries that the U.S. had not done enough to make good on a previous pledge to reduce its nuclear arsenal.

ABC News' Devin Dwyer contributed to this report.