A Serbian court has rejected an extradition request by Montenegro for a suspect in an alleged pro-Russian plot to overthrow the small Balkan country's government.

Nemanja Ristic, the suspect whose extradition was rejected on Thursday by the Special Court in Belgrade, appeared in a group photo with Russian Foreign Minister Sergey Lavrov during his visit to Serbia in December.

Montenegro also has issued extradition requests for another Serb and two Russians for alleged plans to kill the then-prime minister and take over parliament on election day in October.

Montenegro has arrested around 20 people in the alleged plot, most of them Serbian nationals.

The Kremlin has denied involvement, but has actively supported local groups that oppose having Montenegro become the 29th member of NATO.