"This is not some political skirmish that only matters on the American political scene," Tusk told reporters in Washington on Tuesday.
He added: "Mr. Johnson's failure to make a positive decision will cost thousands of lives. He takes personal responsibility for that."
Tusk was speaking to the media after he and Polish President Andrzej Duda held talks at the White House with US President Joe Biden.
During the meeting, Biden and the Polish leaders urged Johnson to move ahead with a vote on a critical aid package for Ukraine, the Reuters news agency reported.
"He must be aware ... that the fate of millions of people depends on his individual decisions, and thousands of lives in Ukraine today and tomorrow depend on his decisions," Tusk said of Johnson.
Biden on Tuesday reassured Polish leaders that American support for their country was "ironclad" amid growing concerns in Europe over Russia's ongoing war in Ukraine.
Former US President Donald Trump, the front-runner for the Republican party's 2024 presidential nomination, opposes aid to Kyiv, according to Reuters.
Russia invaded Ukraine on February 24, 2022, starting the largest armed conflict in Europe since World War II.
Wednesday is day 749 of Russia’s war on Ukraine.
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Source: IAR, PAP, Reuters