File Photo: US Secretary of State John Kerry.

The United States on Monday announced plans to step up the dismantling of retired nuclear warheads and issued a stern warning to North Korea to give up its nuclear program or face deeper isolation.US Secretary of State John Kerry also said world powers were "closer than ever to the good comprehensive deal that we have been seeking" with Iran to curb its nuclear program.Kerry took the podium at the nuclear Non-Proliferation Treaty (NPT) conference to argue that the US administration was committed to advancing disarmament, seeking to deflect criticism that not enough was being done."We are committed to working with you to prove the skeptics wrong over time," said Kerry, stressing that the United States wanted to "leave the race for nuclear arms in the past."Over the last 20 years, the United States has dismantled 10,251 warheads and another 2,500 have been retired and are ready for elimination, Kerry told the conference at UN headquarters in New York."I am pleased to announce today that President (Barack) Obama has decided that the United States will seek to accelerate the dismantling of retired nuclear warheads by 20 percent," he said.Kerry took a swipe at North Korea, saying that Pyongyang must address international concerns about its nuclear program."Until that happens, it will only become more isolated from the rest of the world," he warned.North Korea is already subject to a raft of UN and US sanctions imposed as the result of its three previous nuclear tests, carried out in 2006, 2009 and 2013.Pyongyang is currently believed to have a stockpile of some 10 to 16 nuclear weapons fashioned from either plutonium or weapons-grade uranium.Kerry said the US administration is ready to hold negotiations with Russia on further nuclear strategic cuts -- up to one-third below the level agreed in the 2011 New START treaty.

"That offer remains on the table and we urge the Russians to take us up on it," he said.The month-long NPT conference opened amid much gloom over the slow pace of disarmament led by the United States and Russia, whose relations have been strained over the crisis in Ukraine.