INTERNATIONAL scientists nudged back the minute hand of the symbolic Doomsday Clock,as they praised US President Barack Obama for helping to pull the world back from nuclear or environmental catastrophe.

"It is six minutes to midnight," the Bulletin of Atomic Scientists (BAS), which created the Doomsday Clock in 1947 as a barometer of how close the world is to an apocalyptic end.

"For the first time since atomic bombs were dropped in 1945, leaders of nuclear weapons states are co-operating to vastly reduce their arsenals and secure all nuclear bomb-making material," the statement by the panel of international scientists, including 19 Nobel laureates, said.

"For the first time ever, industrialised and developing countries alike are pledging to limit climate-changing gas emissions that could render our planet nearly uninhabitable."

A key player in the new, global era of co-operation was Mr Obama, whose election in 2008 ushered in "a change in the US government's orientation toward international affairs", said Professor Lawrence Krauss, co-chair of BAS's board of sponsors, speaking at the ceremony to turn back the hands of the symbolic clock.

Mr Obama brought with him to the White House "a more pragmatic, problem-solving approach" than his predecessor, George W. Bush, the scientists said.

"Not only has Obama initiated new arms reduction talks with Russia, he has started negotiations with Iran to close its nuclear enrichment program, and directed the US Government to lead a global effort to secure loose fissile material in four years," Prof Krauss said, reading from the BAS statement.

Since it was created by scientists who helped to develop the world's first atomic weapons, the Doomsday clock has come to be seen as a measure of what progress, if any, the world has made in moving away from the risk of nuclear, climate-caused or bio-warfare catastrophe.

Midnight on the clock signifies the apocalypse, and the minute hand symbolises the countdown to disaster.

The last time the minute hand was moved was in 2007, when Bush was president.

Then, the clock was bumped two minutes closer to midnight.

In resetting the clock this year, the scientists said they were encouraged by recent developments, but had chosen to put back the clock by only one minute to show they were "mindful that the clock is ticking", said Prof Krauss.

"By shifting the hand back from midnight by only one additional minute, we emphasise how much needs to be accomplished" while at the same time recognising that global co-operation has moved forward, he said.

Putting back the clock by only one minute also meant that "there's great potential for it to move again, in either direction", said Prof Krauss.

Which way the hands of the clock are moved next time was up to scientists, world leaders and ordinary people, said Prof Krauss, urging them all to seize the "unique opportunity we have right now to begin to free ourselves from the terror of nuclear weapons and slow drastic changes to our shared global environment.

"We are now poised at a unique time, with hope and opportunity. Let's not blow it," he said.