World leaders have been arriving in Washington ahead of the summit

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. 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.