Russia has informed the United States it will not attend the 2016 Nuclear Security Summit in Chicago.
The international summits -- which aim to enhance nuclear security around the world -- have involved some 50 countries and are a prominent feature of President Barack Obama's foreign policy.
Russia attended the previous summits in 2010, 2012, and 2014.
U.S. State Department spokeswoman Jen Psaki said on November 5 that Russia -- which boycotted a planning meeting for the summit last week -- sent a note stating it would not take part in the summit.
Psaki said Washington regretted the decision but that the United States and Russia "continue to work productively on nuclear issues through other channels."
U.S.-Russian ties are badly strained over Moscow's annexation of Ukraine's Crimean peninsula in March and the conflict in eastern Ukraine.
A Kremlin spokesman declined to comment.