“The Embassy is monitoring reports that extremists have imminent plans to target large gatherings in Moscow, to include concerts, and U.S. citizens should be advised to avoid large gatherings over the next 48 hours," the embassy said on its website.
It issued its warning several hours after Russia's Federal Security Service (FSB), the main successor to the Soviet-era KGB, said it had foiled an attack on a synagogue in Moscow by a cell of the militant Sunni Muslim group Islamic State.
