Six people were killed in a Russian missile attack on an apartment building in the southern Ukrainian city of Mykolaiv early on Friday, Mayor Oleksandr Senkevych said.
Rescuers were digging through the debris for survivors, Senkevych wrote on the Telegram messaging app.
A Reuters reporter in the area heard three impacts, the first at around 3:00 a.m. (0100 GMT).
Photographs posted by Senkevych showed a gaping hole in the multi-storey building and emergency workers combing through a mound of rubble.
Ukrainian forces are steadily advancing further south towards the nearby city of Kherson, from which Russia has said its forces are withdrawing after occupying it since March.
Officials in Kyiv say their troops have reclaimed dozens of settlements in the area.
In an interview with Reuters on Thursday, Ukrainian Defence Minister Oleksii Reznikov said it would take at least a week for Russian troops to leave Kherson.
(Reporting by Dan Peleschuk, Editing by Timothy Heritage)
