North Korea and Iran were cited as particular reasons increasing the danger of a nuclear winter, which had diminished since the end of the Cold War, they said.

The Bulletin of the Atomic Scientists has kept a Doomsday clock at the University of Chicago since 1947 as a reminder of the dangers of nuclear proliferation.

In simultaneous events in London and Washington on Wednesday the clock's big hand was moved forward to five minutes to midnight -- from 11:53 p.m. where it had stood since 2002.

"We stand at the brink of a Second Nuclear Age. Not since the first atomic bombs were dropped on Hiroshima and Nagasaki has the world faced such perilous choices," said the scientists in a statement.

"North Korea's recent test of a nuclear weapon, Iran's nuclear ambitions, a renewed emphasis on the military utility of nuclear weapons, the failure to adequately secure nuclear materials, and the continued presence of some 26,000 nuclear weapons in the United States and Russia are symptomatic of a failure to solve the problems posed by the most destructive technology on Earth."

First set at seven minutes to midnight -- a phrase that has become part of pop culture -- the clock has been moved 17 times in response to global events.

The most recent shift was in 2002 when it moved two minutes forward because the United States withdrew from the Anti-Ballistic Missile Treaty and terrorists were known to be seeking nuclear and biological weapons.

Founded in 1945 by scientists who had helped develop the atomic bomb and were deeply concerned about the use of nuclear weapons, the Bulletin of the Atomic Scientists counts 17 Nobel laureates among its boards of directors and sponsors.

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