Russia plotted to assassinate the prime minister of Montenegro and overthrow its government last year, according to senior intelligence sources in the UK.
An election day coup plot to attack Montenegro's parliament and kill the pro-Western leader was directed by Russian intelligence officers with the support and blessing of Moscow, to sabotage the country's plan to join Nato.
The plot was foiled only hours before it was due to be carried out. It underlines the extreme concern in Russia over the expansion of the transatlantic alliance. Sergei Lavrov, Russia's foreign minister, yesterday criticised Nato as a "Cold War institution" whose expansion had led to unprecedented tensions in Europe over the past 30 years.
The planned coup scheduled for October 16 last year was one of the most blatant recent examples of an increasingly aggressive campaign of interference in Western affairs, UK newspapers have been told. Interpol is now hunting two Russians the Montenegrin government says are intelligence officers who hatched the plot.
The pair spent months overseeing the recruitment and equipping of a small force of Serbian nationalists to attack the parliament building, disguised as local police, and kill Milo Djukanovic, the prime minister.
The Kremlin has strongly denied any involvement, and the Montenegrin special prosecutor has publicly stopped short of alleging Moscow's involvement in a plot it has blamed on "Russian nationalists". But US and UK intelligence agencies which were called in to help unravel the conspiracy are understood to have gathered evidence of high-level Russian complicity. Encrypted phone calls, emails and testimony from plotters-turned-informants are part of the investigation into 21 alleged conspirators.
US Secretary of State Rex Tillerson and his UK counterpart Foreign Secretary Boris Johnson are understood to have discussed the issue last week at their first meeting.
Predrag Boskovic, the Montenegrin defence minister, said there was "not any doubt" that the plot was financed and organised by Russian intelligence.
Sources said the plot appeared to have been constructed so it was deniable and could be blamed on rogue Russian agents and nationalists. But one source said: "You are talking about a plot to disrupt or take over a government in some way. You can't imagine that there wasn't some kind of approval process."
One of the alleged plotters wanted by Montenegrin authorities was recently photographed standing next to Mr Lavrov as he visited Serbia.
Details of the conspiracy have been disclosed after the UK's Defence Secretary Sir Michael Fallon warned of a "step change" in the Kremlin's meddling in Western countries during 2016.
The chief of MI6, Alex Younger, also warned recently that attempts to subvert democracy by states like Russia pose a fundamental threat to sovereignty for Britain and its allies.
The October plot followed repeated warnings from Moscow that Montenegro should abandon plans to join Nato later this year.
Moscow has been keen for the country of 600,000 people to remain inside its own sphere of influence, or at least neutral.
In the months before the coup was due to take place, the Kremlin is suspected of pouring millions of pounds into a slick pro-Russian election campaign run by the country's main opposition bloc, the Democratic Front.
Yesterday the US vice-president tried to reassure European nations worried that Donald Trump intends to sell them out while forging a new relationship with Vladimir Putin.
Mike Pence told European leaders at the Munich Security Conference that the new Trump administration would "hold Russia accountable, even as we search for new common ground which as you know President Trump believes can be found".
Mr Pence said the US would be "unwavering" in its support for Europe, but he reinforced Mr Trump's ultimatum that its countries must spend more on defence.
Yesterday Mr Lavrov told US and European leaders gathered at the Munich Security Conference that Russia desired a "post-West world order."
He said Russia wanted "pragmatic relations, mutual respect, understanding our special responsibility for global stability."
Meanwhile Gen Petr Pavel, who heads Nato's military committee, claimed Russia was behind a false report of a rape by German soldiers in Lithuania that was intended to undermine support for Nato's new eastern force.
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