The Ukrainian military's General Staff said Russia held up to three battalion tactical groups and airborne units in border areas as potential reinforcements.
It said there had been intense shelling of settlements near Kharkiv, in the northeast. On Tuesday, 15 people were reported killed by artillery strikes in the area.
In the Donetsk region: The General Staff said Russian forces were using multiple launch rocket systems "in order to create conditions for the resumption of the offensive on the city of Sloviansk."
Bakhmut: It also said that Ukrainian units had prevented the Russians advancing towards Bakhmut from the south.
The front lines: Elsewhere along active front lines that extend for more than 1,000 kilometers (620 miles), the Ukrainians reported shelling of settlements in Donetsk and Zaporizhzhia regions.
The head of the Donetsk regional military adminstration, Pavlo Kyrylenko, said six children had been wounded and one person killed in shelling of Chasiv Yar. He said cluster munitions had landed at a beach on a lake in the town.