The Doomsday Clock, a symbolic gauge of nuclear danger, has moved one minute closer to midnight because of "inadequate progress" on nuclear and climate issues.
The Bulletin of the Atomic Scientists (BAS) announced the move - to five minutes before midnight - on Tuesday.
The clock last moved one minute back in 2010.
BAS said the failure of multiple nations to control the spread of nuclear weapons was a cause for worry.
The group, which created the clock in 1947, said that two years ago it believed world leaders were trying to address global threat issues.
"In many cases, that trend has not continued or been reversed," it said in a statement to explain the change.
Jayantha Dhanapala, a member of the BAS Board of Sponsors and a former UN undersecretary-general for disarmament affairs, said that while Russia-US nuclear relations were improving, others left much to be desired.
The failure by the US, China, Iran, India, Pakistan, Egypt and Israel to act on the Comprehensive Test Ban Treaty, and by North Korea on a treaty to cut off production of nuclear weapons material "continues to leave the world at risk from continued development of nuclear weapons", he said.
The potential for nuclear weapons use in regional conflicts in the Middle East, Northeast Asia and particularly in South Asia was also alarming, BAS said.
Global climate change was also an issue that needed to be addressed, according to Allison Macfarlane, a BAS Science and Security Board member.
"The global community may be near a point of no return in efforts to prevent catastrophe from changes in Earth's atmosphere," she said in a statement.
BAS called for the adoption of climate change agreements to reduce carbon dioxide emissions and significantly greater investment in renewable energy sources.
The group also said that the disaster at Japan's Fukushima nuclear plant, caused by the 11 March 2011 earthquake and tsunami, "raised significant questions" about nuclear reactor design and oversight.
The decision to change the time was made after a symposium in Washington DC.