John Kerry has officially complained to Vladimir Putin about the incidents

Former ambassadors say it has escalated since Ukraine intervention

One diplomat says a Russian agent defecated on the carpet in his home

Officials told the Washington Post about the most bizarre complaints

John Kerry is fielding an increasing number of complaints from diplomats in Europe, according to former intel officials briefed on secret memos

Russian agents have been going to extreme and peculiar lengths to harass US diplomats and ambassadors in Europe, a report claims.

Frictions between American envoys and their Kremlin-based counterparts are nothing new.

However, a number of US intelligence officials have told the Washington Post that the intimidation techniques have escalated to new heights in recent years - both illegal and bizarre.

On Monday, State Department spokesman Elizabeth Trudeau confirmed John Kerry has officially complained to President Vladimir Putin about the incidents.

One diplomat claims a Russian official broke into his home and defecated on his carpet.

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Complaints: A number of US intelligence officials claim that Russia's intimidation against ambassadors has escalated to new heights since the intervention in Ukraine in 2014. Pictured: the US embassy in Moscow

Others say it is common for Russian intel to break into ambassadors' homes simply to rearrange the furniture.

During President Obama's first term, a Russian agent assassinated an envoy's dog in his home.

And others say they are victims of the 'standard' intimidation tactics - being followed by agents, seeing intel at events they weren't invited to, or having Russia pay media to write negative articles.

The allegations were made in a series of secret memos, seen by John Kerry and other intelligence officials, and described to the Washington Post.

State Department press secretary John Kirby told the Post the complaints have spiked since Russia's annexation of Crimea and intervention in Ukraine.

Two former ambassadors corroborated the claims to the Post.

Norm Eisen, US ambassador to the Czech Republic from 2011 to 2014, told the newspaper: 'Since the return of Putin, Russia has been engaged in an increasingly aggressive gray war across Europe. Now it's in retaliation for Western sanctions because of Ukraine. The widely reported harassment is another front in the gray war.

'They are hitting American diplomats literally where they live.'

Norm Eisen, US ambassador to the Czech Republic from 2011 to 2014 (left), and Michael McFaul, US ambassador to Moscow from 2012 to 2014 (right), described their experiences of intimidation on the job

Michael McFaul, US ambassador to Moscow from 2012 to 2014, told the Post: 'It was part of a way to put pressure on government officials who were trying to do their reporting jobs.

'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.'

Defending Russia's actions, a spokesman for the Russian embassy in DC, told the Post: 'The deterioration of U.S.-Russia relations, which was not caused by us, but rather by the current Administrations’ policy of sanctions and attempts to isolate Russian, had a negative affect on the functioning of diplomatic missions, both in U.S. and Russia.'

Daily Mail Online has contacted the Russian embassy for a comment.