"If Ukraine falls, we will all have very, very serious problems," Tusk told reporters after an EU summit in Brussels.
He said EU leaders had shown a united position on supporting Kyiv amid a change in the approach of the administration of US President Donald Trump.
Tusk also highlighted Ukraine's difficult energy situation, saying the country was struggling with electricity shortages and problems with gas supplies.
Russia likely to escalate 'even before Christmas'
Asked about the possibility of a confrontation with Russia on NATO's eastern flank, Tusk said he did not rely on intuition but on information from Polish, Ukrainian and allied intelligence services.
"All of this adds up to one obvious message: We can expect some escalation even before Christmas," he said.
Tusk said the coming winter would be particularly difficult for Ukraine's railways and infrastructure used to transport gas, oil and food.
"We should expect, as before, but on a larger scale, attempts at sabotage or some provocations in Poland," he said, adding that people in uniform had already been seen on the Latvian side of the border and that Lithuania expected a serious provocation.
Tusk said Russia would "escalate further and test our reaction."
Changes to ETS2
The prime minister also said Poland had secured changes to the EU's ETS2 emissions trading system at the summit, with EU leaders unanimously calling on the European Commission to update its assessment of the system's social and economic impact.
ETS2 is due to extend the EU's emissions trading system to road transport and building heating from 2028. The conclusions adopted by EU leaders call for an updated assessment of the system, including its design and safeguard mechanisms.
Tusk said the changes sought by Poland, other Visegrad Group countries—Slovakia, the Czech Republic and Hungary—as well as Italy and Greece had been included.
"We have the provisions I wanted at this stage, so ETS2 will be reviewed in terms of its social, economic and financial impact, both on people, families, homes and businesses," Tusk said.
He added that he regarded this as only a first step.
"We will seek to postpone ETS2 and perhaps abandon this model altogether. For now, we have knocked out, so to speak, one molar and one incisor," Tusk said.
"I am not satisfied with that, but it is a good start to a review of the whole project," he added.
Tusk also addressed negotiations over the EU's next long-term budget, saying Poland would oppose cuts to funding for cohesion policy and agriculture.
“What mattered most to us has been secured ... Cohesion and agriculture will not be threatened when it comes to European funds, and we certainly will not allow those funds to be reduced,” he said.
Tusk said he had also argued against cuts to European spending on defence and security.
"I said that Poland would not agree and that Europe should in no way accept a reduction in funds for the security and defense of Europe," he said. "I try to explain to them that this is the worst possible time to save money on European spending related to the defence and security of Europe."
Efforts to curb illegal migration
Speaking about migration, Tusk said most EU countries wanted to follow Poland's approach, which he described as a tough asylum policy, tighter border controls and efforts to stop migrants from entering illegally.
He said around 20 EU countries now supported a stronger policy against illegal migration, while Spain and, to some extent, France held different views.
Tusk said he, Italian Prime Minister Giorgia Meloni and Danish Prime Minister Mette Frederiksen were setting the tone of the debate.
Polish PM to visit China
He also said he had told other EU leaders about an invitation he had received from China and that he would make an official visit there later this year.
Tusk said Poland would coordinate its policy toward China with the rest of the EU and would not become China's "Trojan horse" or "fifth column" within the bloc.
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Source: IAR/PAP, TVP Info