Two people were killed and nearly a dozen others were injured in a shoot-out in the city of Mukachevo in the Zakarpattia region of Ukraine's south-west.
Clashes involving firearms started on Saturday, when Right Sector fighters reportedly opened fire at a local sports center after failing to split spheres of influence with a local MP.
When police arrived at the scene, heavy arms, including grenade-launchers and machine guns were used in the ensuing shoot-out, local media reported.
The Ukrainian Right Sector political and military group of radical nationalist organizations has been involved in numerous clashes with police and officials since January 2014.
The ultranationalist party, which is officially banned in Russia, has also taken part in fighting rebels in eastern Ukraine. The extremists have rejected the Minsk ceasefire agreement, having called on President Petro Poroshenko to renew the military offensive in the eastern part of the country