Eight people were killed and four were wounded in Russian artillery shelling in the eastern Ukrainian town of Toretsk in Donetsk region on Thursday, the regional governor said.
The shelling hit a public transport stop where people had gathered, Governor Pavlo Kyrylenko wrote on Telegram. Three children were among the wounded, he said.
Andriy Yermak, the Ukrainian president's chief of staff, said the attack was "another terrorist act" by Russia, and repeated his calls for other nations to declare Russia a state sponsor of terror, a move which would bring further sanctions on Moscow.
Russia has previously denied targeting civilians and has rejected allegations of war crimes in what it calls a "special military operation" aimed at demilitarizing and "denazifying" Ukraine. Ukraine says it was invaded without provocation.
Ukraine said on Thursday it had repelled multiple Russian assaults on a strategic stronghold in the east of the country, and the head of the NATO military alliance said Moscow must not be allowed to win the war.
Ukrainian President Volodymyr Zelenskyy this week described the pressure his armed forces were under in the Donbass region in eastern Ukraine as "hell." He spoke of fierce fighting around the town of Avdiivka and the fortified village of Pisky, where Kyiv has acknowledged its Russian foe's "partial success" in recent days.
The Ukrainian military said Thursday that Russian forces had mounted at least two assaults on Pisky but that Ukraine troops had managed to repel them.
Ukraine has spent the last eight years fortifying defensive positions in Pisky, viewing it as a buffer zone against Russian-backed forces who control the city of Donetsk about 10 kilometers (6 miles) to the southeast.
The Russian Defense Ministry on Thursday confirmed its forces were on the offensive. It said they had inflicted heavy losses on Ukrainian forces around Avdiivka and two other locations in Donetsk province, forcing Kyiv's mechanized and mechanized infantry units to withdraw.
Supplied with sophisticated arms by the West, Ukraine has been targeting Russian-backed forces in the area too.
Officials in the Russian-backed self-proclaimed Donetsk People's Republic (DPR) said on Thursday that Ukrainian shelling had killed at least five people and wounded six in Donetsk city.
NATO warning
Ukraine said the Russian offensive in the east looked like an attempt to force it to divert troops from the south where Kyiv's forces are trying to retake territory and destroy Russian supply lines as a prelude to a wider counter-offensive.
"The idea is to put military pressure on us in Kharkiv, Donetsk and Luhansk over the next few weeks ... What is happening in the east is not what will determine the outcome of the war," Ukrainian Presidential Adviser Oleksiy Arestovych said in an interview on YouTube.
NATO Secretary-General Jens Stoltenberg said on Thursday the war was the most dangerous moment for Europe since World War II and that Russia must not be allowed to win it.
To prevent Moscow from succeeding, NATO and its member countries may have to support Ukraine with arms and other assistance for a long time to come, he said.
"It's in our interest that this type of aggressive policy does not succeed," Stoltenberg said in a speech in his native Norway.
Amid fears among some politicians in the West that Russia's ambitions may extend beyond Ukraine, Stoltenberg warned Putin that the response to such a move from the Western military alliance would be overwhelming.
"If President Putin even thinks of doing something similar to a NATO country as he has done to Georgia, Moldova or Ukraine, then all of NATO will be involved immediately," Stoltenberg said.
The war has led previously non-aligned Finland and Sweden to seek NATO membership, with the request so far ratified by 23 of the 30 member states, including the United States.
Russia, which shares a long land border with Finland, has repeatedly warned Finland and Sweden against joining NATO.