The United States and Britain on Monday for the first time attended a global conference discussing the risks posed by nuclear weapons, reversing their snubbing of previous rounds.

The two countries are permanent UN Security Council members and among the nine states confirmed or believed to possess nuclear weapns, but shunned gatherings held in Norway last year and Mexico in 2014.

Monday's meeting featured some 800 delegates from close to 160 countries discussing the world's 16,300 nuclear weapons.

Other weapons holders present in Vienna were Pakistan and India, but Russia, France and China did not send official delegations, although a Chinese think-tank close to the country's government was present, organisers said.

Other no-shows were North Korea, which has conducted three nuclear tests, and Israel, widely assumed to be the Middle East's only atomic-armed state, although it has never acknowledged it.

The two-day meeting focused on the potential short- and long-term humanitarian consequences of a nuclear explosion, the impacts of nuclear testing and the risks of an accidental atomic blast.

It included Setsuko Thurlow, 82, a wheelchair-bound survivor from the US atomic bombing of Hiroshima, Japan in 1945, who made an impassioned speech warning against the "ultimate evil" of nuclear weapons.

Nuclear weapons have only been used in war twice -- in Nagasaki and Hiroshima -- but there were more than 2,000 test explosions between 1946 and when the practice largely halted in the 1990s.

The largest test by the United States, in Bikini Atoll in the Pacific in 1954, was 1,000 times more powerful than the Hiroshima blast. The USSR's "Tsar Bomba" in 1961 was bigger still.

"As long as nuclear weapons exist, the risk of their use -- deliberately or inadvertently -- remains real. Such a scenario, more than any other human action, has the potential of ending life on this planet as we know it," said Sebastian Kurz, foreign minister of hosts Austria.

Ahead of the conference a group of 118 statesmen and women from 46 countries issued a joint statement warning that the risks posed by nuclear weapons "are under-estimated or insufficiently understood by world leaders".

"In a vestige of the Cold War, too many nuclear weapons in the world remain ready to launch on short notice, greatly increasing the chances of an accident," the statement said.

- Stalled global efforts -

Organisers hope that the Vienna conference will inject some momentum into stalled global moves to reduce the numbers of atomic bombs, five years after US President Barack Obama promised in Prague "concrete steps concrete steps towards a world without nuclear weapons".

Their numbers have fallen sharply from their Cold War highs of some 70,000-80,000, around 90 percent of which are American or Russian, thanks to arms control treaties such as the New Strategic Arms Reduction Treaty (START), the Moscow Treaty and New START.

But there remain around 16,300. Some 4,000 of these are "operationally available", and some 1,800 are on "high alert" and ready for use on short notice, according to the Bulletin of Atomic Scientists.

The follow-through on an "action plan" adopted at a 2010 meeting of signatories to the Nuclear Nonproliferation Treaty (NPT), in particular 22 disarmament steps, has been "very disappointing," said Kingston Reif from the Arms Control Association.

Since New START entered into force in 2011, Russia and the US have failed to commence talks to further reduce their nuclear stockpiles, while progress towards an entry into force of the Comprehensive Test Ban Treaty (CTBT) is stalled, Reif said.

"For all the well-justified praise Obama has received for his efforts to roll back the Iranian nuclear threat, the rest of his nuclear security agenda has stalled," Reif told AFP.

"It stems in large part from forces beyond his control, like Russian aggression in Ukraine.... But a lack of White House attention and an overabundance of caution have also played a role, and if no additional progress is made, Obama's triumphs will not compare favourably to the bold proposals he outlined in 2009," he said.