Volodymyr Zelenskyy says he believes it is time for the creation of an "armed forces of Europe", adding his army was "not enough".
Speaking at the Munich Security Conference, the Ukrainian president said Europe cannot rule out the possibility that "America might say no to Europe on issues that threaten it".
Ukraine has been defending itself from Russia's full-scale invasion for nearly three years.
Mr Zelenskyy noted that many leaders have long spoken about how Europe needs its own military.
"I really believe that time has come," he told the gathering in Germany.
"The armed forces of Europe must be created."
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His comments come after US President Donald Trump held separate calls with Russian President Vladimir Putin and Mr Zelenskyy earlier this week, to try and kickstart a negotiating process to end the war in Ukraine.
Mr Trump then told reporters at the White House that he didn't see any way "that a country in Russia's position" could allow Ukraine to join NATO.
Mr Zelenskyy reacted to Mr Trump's call with the Russian leader by saying the main issue was to "not allow everything to go according to Putin's plan" - after the US president also said it was unlikely Ukraine would get all of its occupied land back.
In doing so, Mr Trump appeared to have conceded two of Mr Putin's war aims before talks had even begun.
"A few days ago, President Trump told me about his conversation with Putin," Mr Zelenskyy said at the conference on Saturday.
"Not once did he mention that America needs Europe at the table. That says a lot," he added.
"The old days are over when America supported Europe just because it always had."
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Following his call with Mr Putin, the US president said the two leaders would likely meet soon to negotiate a peace deal over Ukraine.
Mr Trump later assured Mr Zelenskyy that he too would have a seat at the negotiating table.
"Ukraine will never accept deals made behind our backs without our involvement, and the same rule should apply to all of Europe," Mr Zelenskyy said in Munich.
He went on to say he believed it would be "dangerous" if Mr Trump met Mr Putin before he and the newly-elected US president met.
To Europe's leaders, he issued warnings, asking rhetorically whether their armies would be ready if Moscow launched an open or "false-flag" attack.
The latter is designed to look like it was perpetrated by someone other than the group responsible for it.
"If this [Ukraine-Russia] war ends the wrong way, he [Mr Putin] will have a surplus of battle-tested soldiers who know nothing but killing and looting," he said, citing intelligence reports saying Russia will dispatch its troops to Belarus this summer.
"And now, as we fight this war and lay the groundwork for peace and security, we must build the armed forces of
Europe."
(c) Sky News 2025: Volodymyr Zelenskyy: Ukraine's president calls for creation of 'armed forces of Europe' amid fears of reduction in US support