DONETSK, Ukraine — With street fights and artillery barrages, the Ukrainian military pressed its advance on Wednesday on the two eastern provincial capitals held by pro-Russian separatists in a day of violence that killed 52 civilians and Ukrainian soldiers and an unknown number of rebels.

In one of the heaviest artillery attacks yet on the center of Donetsk, the larger of the capitals, shells struck street kiosks and residential apartment buildings near the stadium of the Shatyorsk soccer club, in the city’s heart. Fighting on the outskirts, particularly around the strategic town of Ilovaysk, a transportation hub, has also flared in recent days.

The fighting has intensified as the Ukrainian and Russian leaders prepare for a meeting in Minsk, Belarus, on Tuesday to explore a diplomatic solution to the conflict, suggesting the sides are maneuvering to achieve the strongest possible military position before then.

In Luhansk, the other remaining separatist stronghold, government forces have now gained control of “significant parts” of the city after days of street fighting, Andriy Lysenko, a spokesman for Ukraine’s National Security Council, told reporters in Kiev.