Montenegro's pro-Russia opposition has vowed to freeze the Balkan country's future membership in NATO if it wins the next parliamentary election, saying upcoming approval of the accession treaty is illegal.

Opposition lawmakers also pledged at an "alternative" assembly session Wednesday in a northern village that if they win the 2021 election they will organize a referendum on Montenegro's status in NATO.

Prime Minister Dusko Markovic has dismissed the opposition declarations as a "show."

Montenegro's parliament is set to ratify the NATO entry treaty on Friday amid opposition boycott and protests.

Montenegro's NATO bid has angered Russia, which views the Adriatic state as a traditional ally. Montenegro has accused Russia of being behind a foiled coup attempt in October aimed at diverting the NATO accession.

Moscow has denied this.