The United States has 5,113 nuclear warheads in its stockpile and "several thousand" more retired warheads awaiting the junk pile, the Pentagon said in an unprecedented accounting of a secretive arsenal born in the Cold War and now shrinking rapidly.
The Obama administration disclosed the size of its atomic stockpile going back to 1962 as part of a campaign to get other nuclear nations to be more forthcoming, and to improve its bargaining position against the prospect of a nuclear Iran.
"We think it is in our national security interest to be as transparent as we can be about the nuclear program of the United States," Secretary of State Hillary Clinton told reporters at the United Nations, where she addressed a conference on containing the spread of atomic weapons.
The United States previously regarded such details as top secret.
The figure includes both "strategic," or long-range weapons, and those intended for use at shorter range.
The Pentagon said the stockpile of 5,113 as of September 2009 represents a 75% reduction since 1989.
A rough count of deployed and reserve warheads has been known for years, so the Pentagon figures do not tell nuclear experts much they did not already know.
Hans Kristensen, director of Nuclear Information Project, Federation of American Scientists in Washington, said his organisation already had put the number at around 5,100 by reviewing budget estimates and other documents.
The import of the announcement is the precedent it sets, Mr Kristensen said.
"The important part is that the US is no longer going to keep other countries in the dark," he said.
Mrs Clinton said the disclosure of numbers the general public has never seen "builds confidence" that the Obama administration is serious about stopping the spread of atomic weapons and reducing their numbers.
The administration is not revealing everything.
The Pentagon figure released Monday includes deployed weapons, which are those more or less ready to launch, and reserve weapons. It does not include thousands of warheads that have been disabled or all but dismantled. Those weapons could, in theory, be reconstituted, or their nuclear material repurposed.
Estimates of the total US arsenal range from slightly more than 8,000 to above 9,000, but the Pentagon will not give a precise number.