SLOVYANSK, Ukraine — Two days after Ukraine’s interim government declared itself all but helpless to control events in the eastern part of the country, Ukrainian military and police forces on Friday resumed their effort to retake this rebel-controlled city, forcing armed separatists from the city’s outskirts with armored vehicles, helicopters and ground troops — and seemingly pushing ever closer to a confrontation with Moscow.

The assault began before dawn, and by early morning church bells sounded a general alarm. With helicopters flying along the city’s edge, troops and armored personnel carriers approached the city from multiple sides. The fighting was brief and then sporadic, however, and clashes subsided by early afternoon, before resuming at night. When movements stopped the Ukrainian troops had advanced, capturing and holding separatist checkpoints and posting infantry and armored vehicles on a bridge overlooking rail lines by the city’s southern border.

Violence also erupted Friday in the previously calmer port city of Odessa, on the Black Sea, where dozens of people died in a fire related to clashes that broke out between protesters holding a march for Ukrainian unity and pro-Russian activists. The fighting itself left four dead and 12 wounded, Ukraine’s Interior Ministry said. Ukrainian and Russian news media showed images of buildings and debris burning, fire bombs being assembled and men armed with pistols.

The deaths expand the increasingly violent struggle for control over Ukraine’s Black Sea port, which had been quiet until last week, when seven people were wounded in a roadside bombing.