Dutch spies reportedly broke into computers used by a Russian hacking group that interfered in the US election, even watching the Kremlin-backed outfit on their own office surveillance cameras.

In an extraordinary counter-espionage operation agents in the Netherlands penetrated the systems of the Russian cyber unit known as Cozy Bear in mid-2014 and monitored them for at least a year.

They collected CTTV footage of individuals involved and pinpointed the location of the Russian group as a university building near Red Square in Moscow.

Dutch operatives informed the CIA and helped remove Cozy Bear from US State Department computers they had hacked into in late 2014.

American spies were reportedly so grateful they sent cake and flowers to their counterparts in the Dutch General Intelligence and Security Service, known as AIVD.

The Dutch also monitored the Russian hackers as they tried to undermine unclassified computer systems in Congress and the White House.

Cozy Bear would later be identified as one of two Russian government-linked hacking groups that broke in to the Democratic National Committee's computers, releasing emails embarrassing to Hillary Clinton ahead of the 2016 US presidential election.

The other Russian hacking group involves was known as Fancy Bear.