The US embassy in Russia warned that “extremists” had imminent plans for an attack in Moscow, hours after Russian security services said they had foiled a planned shooting at a synagogue by a cell from the Afghan arm of the Islamic State.

The embassy, which has repeatedly urged all US citizens to leave Russia immediately, gave no further details about the nature of the threat but 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 US 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, the main successor to the Soviet-era KGB, said it had foiled an attack on a synagogue in Moscow by an IS cell.

It was unclear if the two statements were linked.