The clock was moved to five minutes to midnight, the Bulletin of the Atomic Scientists said, the first adjustment since the beginning of 2010, when it was moved back one minute to six minutes from midnight -- or "doomsday".

"Two years ago, it appeared that world leaders might address the truly global threats that we face. In many cases, that trend has not continued or been reversed," the group said in a statement.

The Bulletin (www.thebulletin.org) is a periodical founded in 1945 by University of Chicago scientists who had helped develop the first atomic weapons in the Manhattan Project.

They created the Doomsday Clock two years later to symbolize how close humanity was to self-annihilation, with an initial setting of seven minutes to midnight.

Initially the clock was focused on nuclear war, but it has been broadened in recent years as the scientists, who include a range of Nobel laureates, added other risks to humanity.

The scientists said world leaders had failed to sustain the progress in nuclear disarmament that had seen them move the hands back on the clock two years ago.

As well, the major global challenge now was a warmer climate that threatens to bring droughts, famine, water scarcity and rising seas, said Allison Macfarlane, an associated professor at George Mason University near Washington, who chairs the group's committee that helps set the clock.

"The global community may be near a point of no return in efforts to prevent catastrophe from changes in Earth's atmosphere," Macfarlane said in the statement.

The last time the group moved the hands closer to midnight was in 2007, by two minutes due to a North Korean nuclear weapon test, Iranian nuclear ambitions and a renewed U.S. emphasis at the time of the military utility of nuclear weapons.

The closest the clock ever came to midnight was 1953, the year of the first test of a hydrogen bomb by the United States.

(Writing by Rodney Joyce; editing by Dan Burns)

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