Poland's president on Sunday said "business as usual" with Russia was impossible after the alleged mass killings of Ukrainian civilians and war crimes blamed on Moscow's forces.
Hundreds of civilian bodies have been found in towns near Kyiv previously occupied by Russian troops such as Bucha and Borodianka.
The southeastern port city of Mariupol lies in ruins after a weeks-long Russian siege that Ukrainian authorities say killed at least 20,000 civilians.
"After Bucha, Borodianka, Mariupol, there cannot be 'business as usual' with Russia," Polish President Andrzej Duda told the Ukrainian parliament in Kyiv, in the first such speech by a foreign head of state since war began on February 24.
"An honest world cannot return to business as usual while forgetting the crimes, the aggression, the fundamental rights that have been trampled on," he added.
Duda expressed regret that some European countries have asked Ukraine to "accept certain demands" from Russian President Vladimir Putin for their economic interests or political ambitions.
Speaking in the presence of Ukrainian President Volodymyr Zelensky, he warned that the most minor concession of Ukrainian territory or sovereignty would be a "big blow" for Ukraine and the West.
"Only Ukraine has the right to decide its future... There cannot be negotiations or decisions taken behind Ukraine's back," Duda said, lauding the country for defending Europe against a "barbarian invasion and the new Russian imperialism".
Duda also underlined "a historic union" between Poland and Ukraine, speaking of the countries' "common future within the European Union" and saying a successful Ukrainian membership bid would be thanks to Warsaw.
Ukraine has applied to join the 27-nation bloc, but leaders including French President Emmanuel Macron and German Chancellor Olaf Scholz have said the process will take time.
Zelensky thanked Duda for "his visit, his support and his true friendship" in an Instagram post.