"The Kiev regime will inevitably collapse if it does not obey the demands by extremists from the Right Sector group and neo-Nazis who joined from the Azov volunteer battalion and similar organizations," Frere said.
According to him, the radicals very unambiguously delivered an ultimatum to the Ukrainian authorities prior to a planned demonstration organized by Right Sector on July 3.
Earlier, the group posted an appeal to the Kiev authorities on its official website, demanding, in particular, that President Petro Poroshenko's regime abandons the Minsk ceasefire agreements and resumes offensive operations in eastern Ukraine.
Right Sector, which was originally strung together from an array of radical nationalist founding organizations, is now a far-right Ukrainian political party that originated as a paramilitary group during anti-government protests on Kiev's Maidan Square in November 2013.
In November 2014, Russia's Supreme Court dubbed Right Sector an extremist organization and banned its activities on Russian territory. Earlier, a criminal case was opened against Right Sector leader Dmitry Yarosh over incitement of terrorism.