The US embassy in Moscow warned Americans late Sunday of potential attacks in public places in Russia, including along border with Ukraine where the Kremlin has massed troops in advance of a feared invasion.
"According to media sources, there have been threats of attacks against shopping centres, railway and metro stations, and other public gathering places in major urban areas," the embassy said in statement.
The mission did not point to specific reports.
The embassy's warning said there was a risk of attacks in Moscow and Saint Petersburg, "as well as in areas of heightened tension along the Russian border with Ukraine".
Western countries have been warning for weeks that Moscow could be planning an attack on its ex-Soviet neighbour, accusing Russia of building up a force of tens of thousands of troops.
The embassy told Americans in Russia to "avoid crowds" and "have evacuation plans that do not rely on US government assistance".
Russia's foreign ministry spokeswoman Maria Zakharova criticised the move on social media, asking whether the US side had followed protocol with the announcement and asking: "What are we to make of this?"
Local authorities around Russia's border with Ukraine in recent days have announced a state of emergency as evacuees cross into the country from rebel-held territory in east Ukraine.
The emergencies ministry said Sunday that more than 50,000 people had entered Russia since separatist leaders in two breakaway republics told women and children to go to Russia Friday, citing an increase in tensions.
Ukraine has been fighting pro-Moscow separatists since 2014 when Moscow annexed the Crimean peninsula in the wake of massive streets protest. More than 14,000 people have since died in the fighting.
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