"Ukraine will not forgive mistreatment of our people, these dead and injured," Zelensky wrote on social media on Monday.
All "Russian terrorists" would be held accountable, the Ukrainian leader asserted.
Zelensky also published a video showing burning cars and debris in Sloviansk.
Sloviansk and Druzhkivka in the Donetsk region to the south were hit and several administrative and office buildings as well as residential buildings were destroyed.
According to authorities, at least two people were killed and another 29 were injured. The front line is almost 25 kilometres from Sloviansk.
