The Pentagon unveiled Monday the size of U.S. nuclear arsenal for the first time, saying it has a total of 5,113 nuclear warheads in its stockpile.

The information was released almost in the meanwhile as Secretary of State Hillary Clinton delivered her address to the Nuclear Non-Proliferation Treaty Review Conference that just kicked off at the United Nations headquarters in New York City.

It was the first time the Pentagon disclosed the number of U.S. nuclear weapons, which has long been treated as classified information.

Secretary Clinton said that the move is aimed at promoting transparency in the nuclear disarmament regime and encouraging other nations to comply with it.

"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," she said.

However, the Pentagon's calculations on its nuclear stockpile contradicted with the estimates of some nonprofit groups.

The United States is estimated to possess some 9,400 nuclear weapons, both deployed and in stock, according to a review published late last year in the Bulletin of the Atomic Scientists.

The United States and Russia are the world's two largest nuclear-weapon states, possessing more than 90 percent of the world's nuclear warheads in combination.

U.S. President Barack Obama and his Russian counterpart Dmitry Medvedev last month signed a successor treaty to the expired Strategic Arms Reduction Treaty (START), agreeing to slash both nations' arsenals of strategic nuclear warheads to 1,550 over seven years, about a third less than currently permitted.