The United States Embassy in Moscow said Thursday that it was aware of "imminent plans" by "extremists" to target public gatherings in the Russian capital over the next two days."The Embassy is monitoring reports that extremists have imminent plans to target large gatherings in Moscow, to include concerts," it said in a security alert issued to American citizens residing in Russia."United States citizens should be advised to avoid large gatherings over the next 48 hours," it added.It was not immediately clear what "reports" the embassy was referring to in its statement.Earlier on Thursday, Russia's Federal Security Service (FSB) said it hadkilled Islamic State militants who were planning a "terrorist attack" on a Moscow synagogue, but it was not clear if the United States Embassy was referring to that incident in its security alert.There was no immediate comment from the Russian government.