KIEV, Ukraine — Ukraine’s security forces pressed their assault to try to reclaim the pro-Russian stronghold of Slovyansk on Saturday, even as the rebels freed seven European military observers and the Kremlin cited the deaths of dozens of people in Odessa as proof that Ukraine could no longer protect its citizens.

The Ukrainian troops built on recent advances into Slovyansk’s outskirts, entering the neighboring town of Kramatorsk after firefights with armed rebels. The Interior Ministry said the forces had recaptured the main state security building there and a television tower for the town, allowing for the resumption of Ukrainian television broadcasts that had been replaced by Russian ones.

But even with the military advances, the violence on Friday in Odessa, far west of the country’s restive eastern region, was a measure of how far events have spiraled out of the authorities’ control. It also added to pressure from Russia, which has long said it could intervene in Ukraine if it believed Russian-leaning residents were in danger. Odessa’s population includes many Russian speakers sympathetic to Moscow.

An official in the city said 46 people had died as a result of street battles between pro-Russia and pro-Ukraine groups; many of the dead were pro-Russia militants who had retreated into a trade union building that was then set on fire. If confirmed, the death toll would be the highest since the struggles in February between pro-Europe demonstrators and the pro-Russia Ukrainian government of the ousted President Viktor F. Yanukovych.