The US has accused Russia of using its intelligence and security services to harass and intimidate diplomats, embassy staff and their families in Moscow and other European capitals.
Officials say the campaign began around two years ago, coinciding with the introduction of sanctions against Russia following the crisis in Ukraine.
Russian agents have allegedly used a range of techniques, from following diplomats and their relatives and turning up uninvited to social events, to breaking into their homes and rearranging the furniture.
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"They are hitting American diplomats literally where they live," said the US State Department's press secretary, John Kirby.
Michael McFaul, the former US ambassador to Russia, told the Washington Post his family was routinely followed and that Russian agents wanted him to know he was being watched.
"It was part of a way to put pressure on government officials who were trying to do their reporting jobs," he said.
"It definitely escalated when I was there. After the invasion of Ukraine, it got much, much worse. We were feeling embattled out there in the embassy."
US Secretary of State John Kerry has spoken to Russian President Vladimir Putin to insist the harassment stops.
A spokesman for the Russian Embassy in Washington says they are acting as a result of "US provocations and mistreatment of Russian diplomats in the United States".
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