MOSCOW — The head of Russia’s military intelligence agency, who oversaw the hacking of the Democratic Party’s computers during the United States’ 2016 presidential election, died on Wednesday after a long illness, Russian state news agencies reported.

Col. Gen. Igor V. Korobov, 63, had not been seen in public for months and was notably absent from a ceremony on Nov. 2 marking the 100th anniversary of the military intelligence agency, known as the G.R.U.

Historically a secretive, little-understood agency, the G.R.U. under General Korobov emerged as Russia’s primary tool of global disruption. Mr. Korobov was sanctioned by the Treasury Department for the release of emails stolen from the Democratic Party in support of Donald Trump’s campaign in 2016.

Under his watch, the G.R.U. was implicated in the theft and publication of documents belonging to the presidential campaign of Emmanuel Macron in France, the hacking of computers of the global antidoping watchdog and the poisoning of a former G.R.U. officer, Sergei V. Skripal, with a highly potent nerve agent in Britain this year.