Polish Prime Minister Donald Tusk has said that there is a "real" threat of conflict between the West and Russia.
The former President of the European Council has warned that Ukraine must not be defeated by Russia for the good of the whole of Europe.
It comes as Ukraine's air force said that Russia fired four missiles into eastern Ukraine overnight, as well as 12 Shahed drones across the country.
Russia has launched an air strike campaign on Ukraine's energy infrastructure over the past week, causing significant damage in several regions.
Donald Tusk has warned about the implications of war breaking out between Russia and the West
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Tusk said that in his part of Europe, war was no longer an abstract idea, warning that "literally any scenario is possible".
He told a press conference alongside his Ukrainian counterpart Denys Shmyhal in Warsaw: "I know it sounds devastating, especially to people of the younger generation, but we have to mentally get used to the arrival of a new era. The pre-war era.
"I don't want to scare anyone, but war is no longer a concept from the past....It is real, in fact it already started more than two years ago."
"We must be ready. Europe still has a long way to go."
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Ukrainian PM Denys Shmyhal with Donald Tusk in Warsaw
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He added: "Today we have to spend as much as we can to buy equipment and ammunition for Ukraine, because we are living in the most critical moment since the end of the Second World War...the next two years will decide everything.
"If we cannot support Ukraine with enough equipment and ammunition, if Ukraine loses, no one in Europe will be able to feel safe."
Over the last week, Russian missile and drone attacks hit thermal and hydroelectric plants in central and western Ukraine.
President Zelensky said the Kaniv hydropower plant was among the targets along with the Dnister plant, which is located on the Dnister River, flowing through neighbouring Moldova.
Polish PM Donald Tusk
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PM Tusk added that Vladimir Putin's attempt to blame Ukraine for the attack on Moscow's Crocus City Hall without evidence was being used for justification for escalation in the region.
He told the press conference that the Russian president: "Evidently feels the need to justify increasingly violent attacks on civil targets in Ukraine."