By NBF News

The US and Russian presidents are at the summit fresh from a new treaty

World leaders at a summit on nuclear security in Washington have heard dire warnings of the danger of nuclear material falling into the wrong hands.

President Barack Obama, opening the biggest international meeting hosted by the US since 1945, greeted leaders from nearly 50 countries.

Officials said more should be done to prevent theft or smuggling.

Meanwhile France's leader stressed his country could not give up its own nuclear weapons.

The US welcomed a Ukrainian pledge to eliminate its stockpile of highly enriched uranium by 2012.

The two-day summit is taking place without representatives of Iran and North Korea, neither of whom were invited by the US because of the disputes over their nuclear programmes.

Israeli Prime Minister Benjamin Netanyahu dropped plans to attend the summit, reportedly because of concern that Muslim states planned to press for Israel to open its own nuclear facilities to international inspection.

'Proliferators not welcome'
The head of the International Atomic Energy Agency, Ikia Amano, said that nuclear powers needed to do more to protect nuclear materials.