The last of the nation's biggest nuclear bombs, a Cold War relic 600 times more powerful than the atomic bomb dropped on Hiroshima, has been dismantled in what one energy official called a milestone in President Barack Obama's mission to rid the world of nuclear weapons.

Workers in Texas separated the roughly 300lb (136kg) of high explosives inside from the special nuclear material – uranium – known as the pit.

The work was done outside of public view for security reasons, but explosives from a bomb taken apart earlier were detonated as officials and reporters watched from less than a mile (1.6 kilometers) away.

The deputy secretary of energy, Daniel Poneman, called the disassembly "a milestone accomplishment." The completion of the dismantling programme is a year ahead of schedule, according to the US Department of Energy's national nuclear security administration, and aligns with Obama's goal of reducing the number of nuclear weapons.

Put into service in 1962, when Cold War tensions peaked during the Cuban Missile Crisis, the B53 weighed 10,000lb (4,500 kg) and was the size of a minivan. Many of the bombs were disassembled in the 1980s, but a significant number remained in the US arsenal until they were retired from the stockpile in 1997....