The clock's keepers see a higher risk of nuclear conflict
The directors of the Bulletin of the Atomic Scientists - a magazine that has campaigned for nuclear disarmament since 1947 - pushed the hands forward by two minutes, to seven minutes to midnight.
They said the move reflected a higher perceived risk of global nuclear Armageddon because of threats including international terrorism and tension between India and Pakistan, both nuclear armed.
The clock was last reset in 1998 after India and Pakistan both carried out missile tests.
The Bulletin's chairman, George Lopez, said the 11 September suicide attacks and evidence that terror groups had tried to acquire nuclear weapons should have been an international wake-up call.
Instead, he said: "The international community simply hit the snooze button rather than raising the general alarm."
Threat factors
Slow progress on nuclear disarmament, a US decision to abandon a key arms control agreement - the Anti-Ballistic Missile (ABM) Treaty - intensified terrorist activity, tension between India and Pakistan and a growing gap between rich and poor all contributed to the increased threat perception, the Bulletin said in a statement.
Of the world's 34,000 nuclear weapons in 1998, only 3,000 have been destroyed in the last four years and thousands remain on alert, ready to be fired in minutes, the statement said.
It said a lack of checks and controls made it impossible to verify whether all nuclear materials in the United States and Russia were accounted for or whether all weapons were secure.
The closest the clock has come to midnight - symbolising the zero hour of an apocalyptic nuclear attack - is just two minutes away.
That was in 1953, when the United States and the Soviet Union tested thermonuclear devices within nine months of each other.
The farthest it has been set from midnight is 17 minutes away.
That was in a wave of optimism at the end of the Cold War, in 1991.
Key treaty
The scientists in charge of the clock say it can only be turned back now if the US and Russia cut their arsenals to 1,000 nuclear warheads each in the next 10 years - and if they increase funding for the protection of nuclear materials and expertise.
The US should also reconsider its plans to turn its back on the ABM Treaty, they said.
The clock was first set in 1947 by scientists at the Manhattan Project - the US effort to develop an atomic bomb during World War II.