Officials already had told the AP on Monday that Moscow was absent from last week's initial summit planning session in Washington but had left unclear whether Russia planned to attend the summit itself.
Two diplomats said on Tuesday that the boycott is applied to the 2016 meeting as well. They cited as their source a diplomatic note from Moscow to US and other nations planning to participate.

The diplomats spoke on condition of anonymity because they are not allowed to discuss confidential information.
One of the diplomats said the note expressed opposition to the summit because of its alleged political nature. He cited the note as saying that any meeting on nuclear security should be on a technical level and convened not by a nation but by the UN's International Atomic Energy Agency.
Russia has participated in such summits in the past. But the diplomat, who is familiar with Moscow's stance, said it already had reservations while attending the last meeting in March at The Hague. He did not specify but added that the changed political atmosphere shorthand for Russia-US tensions over Ukraine added to the Kremlin's decision to stay away.
In Moscow, Russian Foreign Ministry officials did not immediately respond to a request for comment.
At the last summit, 35 countries discussed turning international guidelines on nuclear security into national laws and opening 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.
But Russian reluctance to go along with US backed initiative was already apparent. Moscow refused to back such 35-nation agreement, as did China, India and Pakistan. All four of these nations have nuclear weapons.
The summits were initiated by Obama in 2010 and are aimed at preventing terrorists from getting their hands on weapons-grade nuclear material.

Since the first summit, the number of countries that have enough material to build a nuclear weapon has fallen from 39 to 25.
Outlining his vision in a keynote speech five years ago, Obama announced a new effort to secure sensitive nuclear material within four years and to cripple black-market trade.

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