U.S. and European officials say Russia has boycotted a meeting planning the 2016 Nuclear Security Summit, in a potentially serious blow to U.S.-led efforts to reduce the threat of nuclear terrorism.

It was not immediately clear whether Russia's absence last week meant a planned boycott of the summit itself or if it was a temporary show of displeasure over Washington's condemnation of Moscow's role in Ukraine unrest.

In any case, Russia's no-show is significant. Only three or four planning meetings are scheduled before the spring of 2016, when the summit is tentatively set to open. With Russia one of the world's five recognized nuclear powers, its input is crucial.

President Barack Obama initiated a string of summits in 2010 aimed at preventing terrorists getting their hands on weapons-grade nuclear material.