Tusk urged Western leaders to show solidarity in supporting Ukraine.
He said Europe, the US and the West should show President Vladimir Putin they were ready to stop Russia by even more decisive means, "above all sanctions."
He said Russia bore "100 percent responsibility” for the war, and that anyone suggesting symmetry or blaming Ukraine was "handing Putin a gift."
More voices on both sides of the Atlantic, he said, were arguing for limiting or halting aid to Kyiv in the name of national interest.
"Anyone who seeks to weaken Ukraine in its defence against Russian aggression is acting against their own country’s security and sovereignty," he added.
Tusk said Russia was not seeking a ceasefire, noting that previous attempts at deals, including on halting attacks on energy infrastructure, had ended in Russian strikes.
Ukrainian President Volodymyr Zelensky, who addressed the forum by video link, said Russia had already violated an energy ceasefire announced a day earlier by US President Donald Trump.
Trump said on Truth Social on Sunday that the truce between Russia and Ukraine was taking effect immediately and urged both sides to observe it.
Zelensky, however, told the conference that much of Kyiv was without water and electricity at the time, and that Russia struck a power plant in Kharkiv "immediately" after Trump’s post.
"Putin does not read Truth Social, and his Shahed drones follow a different set of instructions," he said.
"All morning today, more and more Russian jet-powered Shaheds have continued to enter our skies over Kyiv and other Ukrainian cities."
He said Ukraine would observe the truce if Russia did the same, and that Kyiv was ready to sign additional guarantees if Washington considered them necessary.
He thanked Trump for the initiative and said the truce could be an important step towards ending the war if the US ensured Russia respected it.
Asked about Trump’s remark that Ukraine needs a new leader, Zelensky said elections were impossible under constant Russian attacks.
If a ceasefire took hold, he said, he was ready to find "difficult solutions," such as changing the constitution, to hold a vote as soon as possible.
The forum, organised in Warsaw by the Pulaski Foundation, brings together politicians, military officials and security experts.
Norway is this year’s partner country, and its prime minister, Jonas Gahr Støre, is among the guests.
NATO’s top commander in Europe, Gen. Alexus Grynkewich, is due to present his "Theory of Victory" later on Monday.
According to Bloomberg, it combines conventional military strength with new technologies such as artificial intelligence, autonomous systems and drones.
His spokesman said the plan is purely defensive and aims to maintain NATO’s advantage.
(ał)
Source: PAP, IAR
Click on the audio player above for a report by Marcin Matuszewski