The US embassy in Russia has warned that "extremists" have imminent plans for an attack in Moscow.
The warning came hours after Russian security services said they had foiled a planned shooting at a synagogue by a cell linked to Islamic State.
The US embassy, which has repeatedly urged all US citizens to leave Russia immediately, gave no further details about the nature of the threat. However, it said people should avoid concerts and crowds and be aware of their surroundings.
“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.
Russia's Federal Security Service, the main successor to the Soviet-era KGB, said earlier it had foiled an attack on a synagogue in Moscow by a cell of the militant Sunni Muslim group Islamic State.
It was unclear if the two statements were linked.
The FSB said an Islamic State cell was operating in Russia's Kaluga region as part of the Afghan arm of the group, which is known as ISIS-Khorasan.
The group seeks a caliphate across Afghanistan, Pakistan, Turkmenistan, Tajikistan, Uzbekistan and Iran.
It first appeared in eastern Afghanistan in late 2014 and established a reputation for extreme brutality.
The cell "was preparing to attack the congregants of a synagogue using firearms," the FSB said.
When tackled, the militants offered resistance by Russian special forces and were "neutralised" by return fire, it said.
"Firearms, ammunition, as well as components for the manufacture of an improvised explosive device were found and seized," the FSB said.
Relations between the West and Russia were plunged into crisis by the war in Ukraine.
In an address to Congress on Thursday, US President Joe Biden urged lawmakers to continue supporting Ukraine, saying: “Freedom and democracy are under attack both at home and overseas at the very same time. History is watching."
