"In the political turmoil and growing chaos, this is what counts most," Donald Tusk wrote on X.
"Whoever questions this obvious truth contributes to Putin's triumph," he added.
His post came after Trump paused military aid to Ukraine following his clash with Ukrainian leader Volodymyr Zelensky last week, according to reports.
Washington's move comes as Trump shifts US policy on Ukraine and Russia, adopting a more conciliatory stance toward Moscow since taking office in January, the Reuters news agency reported.
"President Trump has been clear that he is focused on peace," a White House official was cited as saying.
"We need our partners to be committed to that goal as well," the official said on Monday, according to Reuters. "We are pausing and reviewing our aid to ensure that it is contributing to a solution."
Poland's top politicians reaffirmed their support for Ukraine after a tense meeting between Trump and Zelensky in the Oval Office of the White House last Friday devolved into a shouting match over Russia's war and Washington's assistance to Kyiv.
Russia launched a full-scale invasion of Ukraine on February 24, 2022, starting the largest military conflict in Europe since World War II.
Tuesday is day 1,104 of Russia's war on Ukraine.
(gs)
Source: Reuters, IAR, PAP