"Do not travel to Moscow, for the next 48 hours..."

The US embassy in Russia has warned that "extremists" have immediate plans to attack Moscow, hours after Russian security services said they had thwarted a planned shooting at a synagogue by a cell of the Afghan branch of the Islamic State.

The embassy, which has repeatedly urged all American citizens to leave Russia immediately, did not elaborate on the nature of the threat, but said people should avoid concerts and crowds and be aware of their surroundings.

"The embassy is following reports that extremists have immediate plans to target large gatherings in Moscow, including concerts, and US citizens should be advised to avoid large gatherings for the next 48 hours," the embassy said on its website.

The UK embassy also issued a public update on the situation, adding: "The US says it is 'following reports that extremists have imminent plans to target large gatherings in Moscow, including concerts, and US citizens should be advised to avoid large gatherings for the next 48 hours.' We advise British citizens not to travel to Russia."

The warning was issued hours after Russia's Federal Security Service said it had foiled an attack on a synagogue in Moscow by a cell of the militant Sunni Muslim group Islamic State.

It is not clear whether the two statements are related.

The war in Ukraine has caused the deepest crisis in Russia's relations with the West since the 1962 Cuban Missile Crisis.

The Kremlin accuses the US of fighting Russia by supporting Ukraine with money, weapons and intelligence.