The US dismantled its last remaining nuclear bomb - one of the most powerful weapons ever built - as part of President Barack Obama's nuclear security policy on Tuesday.

The B-53 bomb - built in 1963, the year of the Cuban missile crisis - was taken apart at the Pantex facility in Amarillo - the only place in the US that still builds, maintains and builds nuclear weapons.

Weighing in at 10,000 pounds and the size of a mini-van, the bomb was built to deliver a 9-megaton blast, around 600 times more powerful than the one that devastated Hiroshima and killed more than 100,000 people in 1945.

"It's significant in the sense that it's the last of these multi-megaton weapons that the nuclear powers used to build during the height of the Cold War," said Hans Kirstensen of the Federation of American Scientists. "This is the end of the era of these monster weapons."

"The world is a safer place with this dismantlement," said Thomas D'Agostino, Under Secretary of Energy and Administrator of the National Nuclear Security Administration.

"The B53 was a weapon developed in another time for a different world."

The process of eliminating the massive nuclear weapons, known as "the last of the big dogs", began 14 years ago. The biggest nuclear bomb in the nation's arsenal now is the 1.2-megaton B83

Last the US revealed for the first time the total size of its nuclear stockpile - a total of 5,113 warheads as of September 30, 2009, the Pentagon announced.

Under a new strategic arms limitation treaty (START) treaty, agreed in April last year, the United States and Russia will drop the number of active strategic nuclear warheads to 1,550 by 2018.