US embassy in Russia said, "U.S. citizens living in or traveling to Russia should leave immediately. Take precautions to avoid the risk of being wrongly detained. Don't travel to Russia."
Hersh, now 85 years old, had won the Pulitzer Prize, considered the highest award in investigative journalism, for exposing the US military's brutal Mai Lai massacre in Vietnam.
NATO and defence leaders from about 50 countries met at the American Ramstein Air Base in Germany on Friday, the latest in a series of arms-pledging conferences since Russia invaded Ukraine 11 months ago.