Russia's President Vladimir Putin said the United States would have to cut its diplomatic staff in Moscow and elsewhere in the country by 755 people and that Moscow could consider additional measures against Washington as a response to new US sanctions approved by Congress.
Russia also said it would seize two US diplomatic properties after the US House of Representatives and the Senate overwhelmingly approved new sanctions on Russia as a response to alleged intervention by the Kremlin in the 2016 US presidential election. The White House said on Friday that US President Donald Trump would sign the sanctions bill.
Putin said in a television interview released on Sunday that the United States would have to cut its diplomatic and technical staff by 755 people by 1 September, and the number of American diplomatic and technical personnel will be capped at 455 - equivalent to the number of their Russian counterparts working in the United States.
Currently, close to 1,200 employees work at the United States' embassy and consulates in Russia, according to US and Russian data. Washington expelled 35 Russians in December.
The United States and Russia have expelled dozens of each other's diplomats before - but Sunday's statement indicated the single largest forced reduction in embassy staff, exceeding anything seen during the Cold War and comparable only to the closing of the American diplomatic presence in the months following the Communist revolution in 1917.
"More than a thousand employees - diplomats and technical employees - have worked and are still working in Russia these days," Putin told journalist Vladimir Solovyov on a nationally televised news show Sunday evening. "Some 755 of them will have to terminate their activity."
Putin's remarks came during a 31/2-day trip by Vice President Pence to Eastern Europe to show US support for countries that have chafed at interference from Moscow - Estonia, Georgia and Montenegro.
"The president has made it very clear that Russia's destabilizing activities, its support for rogue regimes, its activities in Ukraine are unacceptable," Pence said, when asked by reporters in Tallinn, Estonia, whether he expects Trump to sign the sanctions. "The president made very clear that very soon he will sign the sanctions from the Congress of the United States to reinforce that.
"As we make our intentions clear, we expect Russian behaviour to change."
The new US sanctions were partly a response to conclusions by US intelligence agencies that Russia meddled in the 2016 US presidential election, and to punish Russia further for its annexation of Crimea from Ukraine in 2014.
Russia's response suggested it had set aside initial hopes of better ties with Washington under Trump, something the Republican president, before he was elected, had said he wanted to achieve.
A federal law enforcement investigation and multiple US congressional probes looking into the possibility that Trump's campaign colluded with Russia have made it harder for Trump to open a new chapter with Putin. Russia denies it interfered in the election and Trump has said there was no collusion.
The Russian government is also seizing two diplomatic properties - a dacha, or country house, in a leafy neighbourhood in Moscow and a warehouse - following the decision by the Obama administration in December to take possession of two Russian mansions in the United States.
The State Department declined to comment on the exact number of embassy and consular staff in Russia.
It is not yet clear how the State Department will reduce its staff in Russia. Some of the local staff was hired to help with a significant expansion of the US embassy compound in Moscow.
After the State Department, the next largest agency presence in Moscow in the 2013 review belonged to the Defense Department, which had 26 employees working for the Defense Intelligence Agency (20 of them US nationals) and 10 working for the Defense Threat Reduction Agency (nine of whom were US nationals).
The Library of Congress had two US staff and two foreign staff, and space agency NASA had eight US staff and four foreign staff members. There were 24 Marine security guards.
The move increases the likelihood of new, perhaps asymmetrical reprisals by the United States in coming days.