KIEV, Ukraine—An assault by members of a far-right party on the head of state television here has sharpened the focus on one of the new Ukrainian government's biggest liabilities: the inclusion of onetime fringe nationalists.

The Svoboda party's presence in the ruling coalition—it has four cabinet seats, including defense minister and a deputy prime minister—has fueled accusations by Russia that the new government, backed by Europe and the U.S., includes fascists who pose a threat to ethnic Russians.

The Kremlin has used this alleged threat in a vague way as justification to annex Ukraine's heavily Russian Crimea region, and to approve military action in Ukraine.

The attack late Tuesday at Ukraine's First National television, which was filmed by the Svoboda members themselves and posted online, plays into the Kremlin narrative, which thousands of more moderate, pro-Europe protesters in Kiev have called a smear.

The recording shows three members of parliament from Svoboda entering the network offices with several other supporters. Upset by recent coverage they deemed too pro-Russian, they manhandled the acting director, Oleksandr Panteleymonov, cursed him and forced him under threat of violence to sign a letter of resignation