Russia has told the US it will boycott the 2016 Nuclear Security Summit, according to diplomats, damaging efforts initiated by President Barack Obama to reduce the worldwide threat of nuclear terrorism.

Officials said yesterday that Moscow was absent from last week's summit planning session in Washington but left unclear whether Russia planned to attend the summit itself.

Two diplomatic sources today cited a diplomatic note from Moscow saying the boycott would apply to the 2016 meeting as well.

One of the diplomats said the Russian note expressed opposition to the summit because of its alleged political nature, and said that any meeting on nuclear security should be on a technical level.

Mr Obama initiated a string of summits in 2010 aimed at preventing terrorists from getting their hands on weapons-grade nuclear material. Since then, the number of countries that have enough material to build a nuclear weapon has fallen from 39 to 25.

At the last summit this year in The Hague, 35 countries pledged to turn international guidelines on nuclear security into national laws and open up their procedures for protecting nuclear installations to independent scrutiny.

The summit also featured new reduction commitments, with Japan, Italy and Belgium agreeing to cut their stocks of highly enriched uranium and plutonium.

At the same time, there were setbacks. Russia was notably absent from the 35-nation agreement, along with China, India and Pakistan - all nations with nuclear weapons.

The officials said that with the exception of Russia, all of the 54 countries that participated in the March summit attended the preparatory meeting in Washington.

Patrick Ventrell, a spokesman for the National Security Council at the White House said the US regretted the Russian decision not to attend.