Full coverage: Xi Visits Czech Republic, Attends Nuclear Security Summit

A notable absence from the Nuclear Security Summit is Russia. The United States says Russia risks further isolation by refusing to participate. But the Kremlin says while nuclear safety is an important issue, there are much bigger problems keeping them away.

The Nuclear Security Summit in Washington will bring together world leaders on a mission to reinforce their countries' commitments to securing nuclear technologies and materials.

Russia has attended three previous summits; in Washington DC in 2010, in Seoul in 2012 and in The Hague in 2014.

However at the end of the last summit Russia said it had already decided it would not participate in the next one.  

Some countries interpreted that decision as a reaction to increased tensions with the United States. Russia's also been accused of fencing itself off from direct dialogue on nuclear safety at a time of increasing threats of nuclear terrorism. 

But Moscow maintains that while previous summits were useful in forging an international consensus on nuclear security and raising awareness of the nuclear terrorism threat, they've now run their course.

Russian experts say Russia's absence from the security summit does not mean that it intends to turn away from its previous summit commitments. Nor, they say, is it planning to give up its obligations under the United Nations nuclear body - the IAEA, or the Global Initiative to Combat Nuclear Terrorism.

Russia says the Nuclear Security Summit, however well-intentioned, has become a source of interference for the larger bodies whose work should be paramount most significantly, the IAEA. And for that reason, Russia says it can no longer be a part.