"Putin wants to go as far as we allow him - not only Ukraine, but the whole of Europe," Poroshenko said in an interview with French radio station RFI.
"If you had asked me two years ago, I would have said this was impossible because the whole postwar security system wouldn't allow that," he said.
"But if you asked me today... Unfortunately everything is possible because the annexation of Crimea and the eastern part of Ukraine... Shows we can break the global security system."
"Is an attack on Finland possible? Yes, and Finland knows that. Is an attack on the Baltic states possible? Yes," he said.
"When we're talking about the fighting in the east of my country... We're fighting not only for the independence and sovereignty of my country, we're fighting for democracy, we're fighting for freedom and the security of the whole European continent.