The Nuclear Security Summit in the Hague will form the backdrop for an emergency meeting of Group of Seven leaders on Russia's annexation of Crimea. It is a confrontation between Russia and the West reminiscent of the Cold War.
Russian President Vladimir Putin is not attending, instead sending Foreign Minister Sergey Lavrov, who is expected to hold talks with US Secretary of State John Kerry.
But experts say frantic diplomacy focused on Ukraine should not divert from the goal of better security of nuclear material.
Delegations from 53 countries, including the leaders of the US, China and Japan, have started to arrive in the Hague. They will meet to negotiate on reducing and securing supplies, and keeping them out of terrorists' hands. The G-7 includes the US, Britain, Germany, Japan, France, Italy and Canada.
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Notable absentees from the summit are North Korea and Iran, excluded by mutual consent.
The summit is the third since US President Barack Obama launched the series in 2009 shortly after taking office, saying that reducing the risk of a terrorist attack with either a nuclear weapon or a "dirty bomb" was one of his most important international policy goals.
Still, they already have reached tangible results. The number of countries that possess enough highly enriched uranium or plutonium to make a bomb has fallen steadily, from 39 before the first conference in Washington in 2010 to 25 at the start of the Hague summit.
Piet de Klerk, the Dutch diplomat who chaired negotiations before the summit, says a further large drop in numbers is unlikely.
But he says The Hague closing accord will likely include a new commitment to reduce plutonium stores. That is particularly relevant for Japan, which owns a large plutonium stockpile even as it reevaluates its use of nuclear power in the wake of the 2011 Fukushima disaster.