Montenegro's higher court on Thursday confirmed prosecution indictments against 14 people, including two Russians charged with masterminding a coup attempt aimed at preventing Montenegro from joining NATO.

Russian nationals Eduard Shishmakov and Vladimir Popov have been indicted with various criminal offenses, terrorism and acts against the constitutional order of Montenegro, a court statement said.

The two alleged members of Russia's military security agency had reportedly operated from neighboring Serbia with sophisticated spying equipment. They have returned to Russia and are beyond the reach of Montenegro's judiciary.

Shishmakov was a deputy military attaché at the Russian embassy in Warsaw, but was declared persona non grata in Poland in June 2014 because it was believed that he was involved in spying.

The other 12 suspects, mostly Serbs, allegedly planned on the election day in October to take over parliament in the capital of Podgorica and kill then-Prime Minister Milo Djukanovic.

Among the indicted suspects are two top Montenegrin opposition officials who have made frequent visits to Moscow before and after the alleged coup attempt.

"This is a political process against fierce opponents of NATO," said opposition leader Milan Knezevic, who was indicted. Montenegro, once a Russian ally, formally became the 29th member of NATO on Monday, despite Moscow's strong opposition.