WASHINGTON — Russia has informed the United States that it is planning to reduce its participation next year in a joint effort to secure nuclear materials on Russian territory, a move that could seriously undermine more than two decades of cooperation aimed at ensuring that nuclear bomb components do not fall into the hands of terrorists or a rogue state.

Sergey V. Kirienko, the head of Russia’s state nuclear company, has told senior Obama administration officials that no new projects in Russia are “envisioned” in 2015, according to American officials.

The officials still hope to persuade the Russians to continue work next year on some current projects, though Russian officials have yet to agree.

The reduced cooperation is a byproduct of the general downturn in relations between Russia and the United States, which has been compounded by President Vladimir V. Putin’s decision to intervene militarily in Ukraine. But it also stems from longstanding concerns among Kremlin hard-liners about a program that brings American nuclear experts to Russia’s nuclear sites and that, they fear, may create the impression that Russia is in need of outside help.