**Leaders from more than 40 countries are gathering in the US for a summit called by President Barack Obama to look at measures to improve nuclear security.**It is a major plank of Mr Obama’s nuclear disarmament and non-proliferation agenda.
Ahead of the two-day Washington summit, he said groups like al-Qaeda would not hesitate to use nuclear devices.
Israel, India and Pakistan, which have not signed the non-proliferation treaty, are attending the summit.
Part of the thinking behind this gathering is to get an international consensus on the scale and nature of the threat, and to try to reduce the risk of it happening, says the BBC’s Nick Childs.
It is estimated there are around 1,600 tonnes of highly enriched uranium in the world - the type used in nuclear weapons - and that virtually all of it is held by the acknowledged nuclear weapons states, most of it in Russia.
There are also about 500 tonnes of the other key ingredient of a nuclear weapon - plutonium.
In total, that is enough to make 120,000 nuclear weapons.
There is another concern: less potent nuclear material to make what is known as a “dirty bomb” that could cause devastating contamination if not actual destruction in a major city, our correspondent says.
Much international, largely US-funded, effort has attempted to clamp down on the threat of nuclear leakage from Russia in particular, but it remains a concern.
One key particular focus at the moment is nuclear security in Pakistan.
Syria left out
Leaders or other representatives of 47 states are attending the two-day summit in Washington.
India, China and Pakistan are among those coming to Washington for one of the biggest gatherings of world leaders in the US in decades.
Neither North Korea nor Iran, two states with disputed nuclear ambitions, have been invited to the summit. The two countries are viewed by the US as violators of the non-proliferation agreement.
Syria was also left off the invitation list because the US believes Damascus has nuclear ambitions, the Associated Press news agency notes.
Israel is being represented at the summit only by a deputy prime minister amid reports that its government is worried that Turkey and Egypt might use the occasion to raise the issue of its nuclear arsenal.
Ahead of the summit, President Obama is holding bilateral meetings with a handful of international leaders.
Last week, the US and Russia signed a new Strategic Arms Reduction Treaty, reducing each country’s deployed nuclear arsenal to 1,550 weapons.
Earlier in the week, Mr Obama approved a new nuclear policy for the United States, saying he planned to cut the nuclear arsenal, refrain from nuclear tests and not use nuclear weapons against countries that did not have them.This article is from the BBC News website. © British Broadcasting Corporation, The BBC is not responsible for the content of external internet sites.