The US ambassador to Russia on Wednesday called on American citizens to either leave Russia or think of a plan on how to stay in the country without the embassy's help amid concerns about the possible closure of the US mission by the Russian government.

Speaking at a press briefing in Moscow, John Sullivan voiced concern that the Russian government may close the US diplomatic mission.

"Our recommendation is as follows: leave Russia now. If you are thinking of leaving, if you can leave, leave now. Or if you are not leaving now, think of a plan for how you will stay here without help from the embassy, because the Russian government may close the embassy," he said.

The US has been a powerhouse of sanctions against Russia amid the Russia-Ukraine war, which has drawn international condemnation, led to financial sanctions on Moscow, and spurred an exodus of global firms from Russia, including McDonald's and Starbucks.

At least 474 civilians have been killed and 864 others injured in Ukraine since Russia launched a war against the Eastern European country on Feb. 24, according to UN figures, with the real toll feared to be higher.

More than 2.1 million people have fled Ukraine to neighboring countries, according to the UN refugee agency.