Russian President Vladimir Putin has announced that 755 staff must leave US diplomatic missions, in retaliation for new US sanctions against Moscow.
The decision to expel staff was made on Friday, but Mr Putin has now confirmed the number who must go by 1 September.
This is thought to be the largest expulsion of diplomats from any country in modern history, says the BBC’s Laura Bicker in Washington.
Staff in the embassy in Moscow as well as the consulates in Ekaterinburg, Vladivostok and St Petersburg are affected, the BBC’s Sarah Rainsford in Moscow adds.
The US said the move was a "regrettable and uncalled for act".
"We are assessing the impact of such a limitation and how we will respond to it," a state department official said.
Mr Putin did strike a conciliatory note, saying he did not want to impose more measures, but also said he could not see ties changing "anytime soon".
In December, the Obama administration ordered the seizure of two Russian diplomatic compounds and expelled 35 Russian diplomats in response to alleged hacking of the US Democratic Party and Hillary Clinton’s campaign.