Russia's defense ministry said on Sunday that its forces had destroyed Ukrainian surface-to-air missile systems near Kharkiv and elsewhere in the east.
Satellite imagery released by US firm Maxar Technologies shows a large convoy of armored vehicles and trucks moving south. The images were collected about 80 kilometers east of Kharkiv. The area is believed to be under Russian control.
Ukrainian presidential adviser and negotiator Mykhailo Podolyak has suggested there could be large-scale battles in eastern Ukraine, and his country is ready.
Podolyak said on Ukrainian television on Saturday that his country is ready for big battles. He said Ukraine must win them, including in the eastern region of Donbas. He added that Ukraine will have a substantive negotiating position once that happens.
Podolyak also suggested the two countries' presidents will meet after that, adding it could take two or three weeks.
BBC and other media outlets quoted sources as saying Russian General Alexander Dvornikov has been tapped to lead Russia's operations in Ukraine.
Dvornikov is believed to have been appointed to reorganize Russian forces after their withdrawal from Kyiv to prepare for a major offensive in eastern Ukraine.
Dvornikov, the commander of Russia's southern military district, has previously headed military operations in Syria.
The administration of President Vladimir Putin began intervening in Syria's civil war in 2015. Many civilians have been killed in that conflict.
Russian forces are intensifying their offensive in eastern Ukraine.