US President Barack Obama welcomes world leaders to Washington today for a summit on nuclear security that aims to refocus attention on an issue he has called a top priority but on which his Administration has had limited success.

Seven years after he envisioned "a world without nuclear weapons" during a high-profile speech in Prague, Obama enters the last of four nuclear summits having proposed deep budget cuts next year on programmes to stop nuclear proliferation while leaving intact military spending on a new generation of weapons.

Countries that have not given up stockpiles of nuclear material include the riskiest ones. And this week's summit will have a glaringly empty chair: Russia has chosen not to attend amid tensions with the US.

"The President has only accomplished a fraction of what he hoped to achieve," said Joseph Cirincione, president of the Ploughshares Fund.


Obama has won a landmark agreement to limit Iran's nuclear programme, has persuaded about a quarter of the countries with loose nuclear materials to move them off their soil and signed with Russia a new START treaty that includes new weapons limits.

White House officials defended the Administration's track record on nuclear security and disputed the suggestion that Russia's absence is a significant setback.

Deputy national security adviser Ben Rhodes noted Russia's support of the nuclear pact with Iran and said Moscow's decision not to participate in the summit is a "missed opportunity for Russia above all".