Jakub Kumoch made the assessment in an interview with Polish state news agency PAP on Monday.
Earlier in the day, Russia fired dozens of missiles on cities across Ukraine, from the capital Kyiv to Lviv in the west and Kharkiv in the east, among other urban centres, British broadcaster the BBC reported.
In Kyiv, at least eight civilians have been killed and 24 injured in the blasts, according to officials.
Valery Zaluzhny, the Commander-in-Chief of the Armed Forces of Ukraine, said on Telegram that Russian forces had fired 75 missiles at Ukraine on Monday morning, of which Kyiv’s air defences had neutralised 41, as cited by Ukraine’s digital broadcaster espreso.tv.
"The Armed Forces are doing everything possible to protect fellow citizens. However, the enemy attack continues. I call on everyone to stay in shelters," Zaluzhnyi added.
‘State terrorism’
Poland’s Kumoch, who serves as senior foreign-policy aide to President Andrzej Duda, said of Russia’s attacks: “We are dealing with state terrorism and a crime being comitted by Russia.”
Jakub Kumoch.
He added: “If anyone had any doubts about whether Russia was committing crimes in this war, then today these doubts have probably been dispelled.”
The official said that Russia had timed its attacks on civilian targets to inflict maximum casualties.
“This bears all the hallmarks of a crime,” he added.
Monday’s attacks on Ukrainian cities took place a day after Russian President Vladimir Putin accused Ukraine of “an act of terrorism” against the Kerch Strait bridge linking Russia and Crimea, prompting calls for reprisals from top officials in Moscow, the Reuters news agency reported.
Kumoch commented that Monday’s strikes reflected “a certain kind of helplessness and revenge for the fact that Russia is suffering defeats in this war.”
He added that helplessness was “driving Russia to engage in criminal activity.”
Kumoch stressed that accusing Ukraine of destroying the Kerch Strait bridge was “insufficient justification for bombing civilian targets.”
“No justification can ever be sufficient” to kill civilians, he stated, adding that Russia was committing "a war crime or even a crime of genocide."
Monday is day 229 of the Russian invasion of Ukraine.
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Source: PAP, global.espreso.tv, bbc.com