Russian shelling set a central market ablaze in Ukraine's eastern city of Slovyansk, killing two people and injuring seven, an official said.
"This is pure terrorism!" Donetsk regional head Pavlo Kyrylenko said.
Mayor Vadym Lyakh said Slovyansk was being hit by Russian artillery salvoes from closer positions, as it became a frontline city.
He posted photos of Tuesday's blaze on Facebook, describing massive shelling, and urging people to stay in shelters.
Russia is seeking to capture all of the eastern industrial Donbas area.
The area is made up of two regions, Luhansk and Donetsk. After recognising two rebel self-proclaimed statelets there, Russian President Vladimir Putin ordered an invasion of Ukraine on 24 February.
Last week, Ukraine's military said it had to pull out of Lysychansk, its last remaining stronghold in Luhansk, under intense Russian fire.
It said its troops were now strengthening fortifications to defend areas it holds in Donetsk, including Slovyansk.
Luhansk regional head Serhiy Haidai on Tuesday said that "heavy fighting is going at the edge of the Luhansk region near Lysychansk".
"The enemy has deployed significant forces," he said, adding that Russia was trying to send more of its forces across a river by building pontoon bridges.
Mr Haidai said the Russians were "sustaining heavy losses".
The BBC has not been able to verify details of the latest attacks.
