European Commission President Ursula von der Leyen confirmed on Sunday that the European Union is also shutting down EU airspace to Russia.
“US citizens should consider departing Russia immediately via commercial options still available,” said a security alert dated 27 February posted on the web site of US Embassy in Moscow.
It also asked American citizens to have “a contingency plan that does not rely on US government assistance.”
A string of European countries, including Germany, France and Italy, had already closed their airspace to Russian airlines and aircraft.
President Volodymyr Zelensky told Boris Johnson in a phone call on Sunday evening that the next 24 hours would a “crucial period” for Ukraine.
The British prime minister promised Mr Zelensky he will do “all he could” to ensure further military aid reaches Ukrainian forces, who were involved in further heavy fighting at the weekend.
Earlier on Sunday, Vladimir Putin placed Russia’s nuclear deterrence forces on high alert, citing aggressive statements by Nato leaders and economic sanctions against Moscow over its invasion of Ukraine.
A Ukrainian delegation is due to meet the Russians “without precondition” on Monday on the border with Belarus, although Mr Zelensky has indicated he sees little prospect of any meaningful dialogue.
The EU also announced over the weekend that it would fund arms supplies to Ukraine and ban pro-Kremlin media.
It also said it would supply combat jets to the Ukrainian armed forces.
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