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"A source close to the Belgrade government said Patrushev, a former FSB (federal security service) chief, apologised for what he characterised as a rogue operation that did not have the Kremlin's sanction," The Guardian wrote on November 11.

Serbia has deported a group of Russians suspected of involvement in a coup plot in neighbouring Montenegro, the Guardian has learned, in the latest twist in a murky sequence of events that apparently threatened the lives of two European prime ministers.

Read alsoMontenegro PM pro-Russian opposition plotted to kill him, demands explanations from RussiaThe plotters were allegedly going to dress in police uniforms to storm the Montenegrin parliament in Podgorica, shoot the prime minister, Milo Djukanovic, and install a pro-Moscow party.

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Diplomatic sources told the Guardian the Belgrade government quietly deported the Russians after the intervention of Patrushev, who flew to Belgrade on October 26 in an apparent effort to contain the scandal. The country's interior minister, Nebojsa Stefanovic denied the government carried out any deportations connected to the plot.

"A group of 20 Serbians and Montenegrins, some of whom had fought with Moscow-backed separatists in eastern Ukraine, were arrested in Podgorica, the Montenegrin capital. In Serbia, meanwhile, several Russian nationals suspected of coordinating the plot were caught with EUR 120,000 and special forces uniforms," The Guardian wrote.

According to the Belgrade daily, Danas, the Russians also had encryption equipment and were able to keep track of Djukanovic's whereabouts.

The Serbian government was further rattled three days after Patrushev's visit when a cache of arms was found near the home of the prime minister, Aleksandar Vucic. The weapons were discovered at a junction where Vucic's car would normally slow down on his way to the house.

Stefanovic said there were "strong suspicions" that an organised crime gang had been hired to kill Vucic for EUR 10 million, but he would not specify who was behind the alleged plot, saying further investigation would show whether people "outside the region" were involved.