Ukrainian President Volodymyr Zelenskyy has there would be no let-up in the counteroffensive that has reclaimed northeastern towns and cities.
On Monday, Kyiv said that its troops had secured the eastern bank of the Oskil River and were preparing for an assault on Russia's forces in the Donbas region.
The river, which flows south from Russia into Ukraine, had been a natural break in the newly emerged front lines.
In recent days, the pace of the Ukrainian advance has again slowed, but Zelenskyy said this was only because the forces were consolidating and preparing for further offensives.
“Maybe now it seems to some of you that after a series of victories we have a certain lull,” Zelenskyy said in his nightly video address on Sunday.
“But this is not a lull. This is preparation for the next series... Because Ukraine must be free — all of it.”
Russia has also accused Ukraine of killing 13 civilians in artillery shelling on Monday in the separatist-held city of Donetsk.
A Russian missile struck close to a nuclear power plant in southern Ukraine on Monday, Kyiv has claimed.
The strike did not damage the three reactors at the Pivdennoukrainsk nuclear plant but reportedly hit other industrial equipment. Ukraine’s presidential office said the attack also severed three power transmission lines.
The plant -- also known as the South Ukraine Nuclear Power Plant -- is Ukraine's second-largest.
Ukraine’s Ministry of Defence and the plant's operator Energoatom have both accused Russia of committing an act of "nuclear terrorism".
The strike follows warnings from Russian President Vladimir Putin of possible stepped-up attacks on key Ukrainian infrastructure.
Putin claimed on Friday that his forces have until now acted with restraint against
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