U.S. has 5,113 nuclear warheads.

Reuters) - The United States disclosed for the first time on Monday the current size of its nuclear arsenal, lifting the veil on once top-secret numbers in an effort to bolster non-proliferation efforts.

The Pentagon said it had a total of 5,113 warheads in its nuclear stockpile at the end of September, down 84 percent from a peak of 31,225 in 1967. The arsenal stood at 22,217 warheads when the Berlin Wall fell in 1989.

The figure includes warheads that are operationally deployed, kept in active reserve and held in inactive storage. But it does not include "several thousand" warheads that are now retired and awaiting dismantlement, the Pentagon said.

"The United States is showing that it is being increasingly transparent," a senior U.S. defense official told reporters at the Pentagon.

"It's part of our commitment ... to set the stage for strength in non-proliferation and for further arms control."

The official declined to offer the Pentagon's estimate for Russia's arsenal and renewed calls for greater transparency by China, saying there was "little visibility" when it came to Beijing's nuclear program.

The United States is also pushing for a new round of sanctions against Iran over its nuclear program.

By releasing the data during the nuclear Non-Proliferation Treaty review conference, analysts said the United States was trying to show it is cutting its arsenal so as to help persuade other states to tighten the global non-proliferation regime.

"It is hugely important for the United States to be able to say, 'Look we are living up to our obligations under the NPT," said Hans Kristensen, director of the Nuclear Information Project at the Federation of American Scientists.

The disclosure comes less than a month after President Barack Obama unveiled a new policy restricting the U.S. use of nuclear weapons and signed a landmark arms reduction accord with Russia.

Obama, who won a Nobel Peace Prize in part for his vision of a nuclear free world, has also renounced the development of new atomic weapons.

Historically, the overall size of the arsenal has been kept secret to help prevent potential adversaries from using the information to more precisely neutralize U.S. nuclear forces.

Still, analysts warned the disclosure could also negatively impact perceptions of the United States -- possibly dismaying other nations by demonstrating how many nuclear weapons it retains two decades after the Cold War ended.

"I think the states that are most concerned about nuclear disarmament will be more focused on the number that remain rather than the number (reduced)," said George Perkovich, director of the Nuclear Policy Program at the Carnegie Endowment for International Peace.

The Pentagon said from fiscal years 1994 through 2009, the United States dismantled 8,748 nuclear warheads.

The Pentagon also declined to disclose the exact number of warheads awaiting dismantlement. It said more analysis needed to be done to make sure it did not impact U.S. national security.

The United States aims to dismantle those warheads by the early part of the next decade, another U.S. official said, also briefing reporters on condition of anonymity.

(Editing by Cynthia Osterman)