VIENNA (AFP) - The US and Britain took part Monday for the first time in a conference of some 800 delegates from more than 150 countries exploring the risks posed by the world's 16,000 nuclear warheads.

The two countries, out of nine nations believed to have nuclear weapons, had shunned two earlier gatherings in Norway last year and in Mexico this March.

Also present in Vienna from the nine were again Pakistan and India, but absent were Russia, France and China, 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 will focus on the potential short- and long-term consequences of a nuclear explosion, the impacts of nuclear testing and the risks of an accidental atomic blast.

Organisers hope it will also inject some momentum into troubled global moves to reduce the numbers of nuclear weapons ahead of a May 2015 conference to review progress implementing the Nuclear Nonproliferation Treaty (NPT).

"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.

"There could not and would not be a winner in such a scenario. We have the collective responsibility for ourselves and future generations to do our utmost that they will never be used again," he said.