On Saturday night, a Russian rocket struck an apartment building in the eastern Ukrainian town of Chasiv Yar, killing 15 people.
Local officials on Sunday expressed fears that more than 20 people may still be trapped in the rubble.
Pavlo Kyrylenko, governor of the Donetsk region that includes Chasiv Yar, told reporters that the town of about 12,000 was hit by Uragan rockets fired from truck-borne systems.
According to reports, Chasiv Yar is about 20 kilometers (12 miles) southeast of Kramatorsk, a city expected to be a primary target of Russian forces as they grind westward.
The Donetsk region is one of two provinces, along with Luhansk, that makes up the Donbas region, where separatist rebels have fought Ukrainian forces since 2014.
Just last week, Russia captured the city of Lysychansk, the last major stronghold of the Ukrainian resistance in Luhansk.