Boris Bondarev, whose LinkedIn says he worked at the Russian mission to the UN in Geneva, told the BBC he knew his decision to speak out may mean the Kremlin now considers him a traitor.
But he stood by his statement which described the war as "a crime against the Ukrainian people" and "the people of Russia".
Moscow has not yet commented.
Russia has cracked down on those who are critical of or veer from the official narrative surrounding the war, which it refers to only as "a special military operation".