"It is absolutely the same situation like the Russian bombardment in Aleppo," he told CNN's Christiane Amanpour in an exclusive interview Wednesday.
"They have only one purpose – [the] world should be less stable, less secured."
A ceasefire deal reached last year in Belarus – the so-called Minsk agreement – is a distant memory in eastern Ukraine, where CNN witnessed intense fighting in the last several days.
Unrest there has been on the rise, and Russia has recently conducted military drills in Crimea, the peninsula it annexed from Ukraine in 2014 – a move most nations do not recognize as legitimate.
Had you asked him in 2013, Poroshenko said, if it would have been possible for Russia to "occupy Crimea," he would have said "no, this is not possible – there is some red line, and Putin [will] not cross this line."
"If you asked me in January, year 2014," he went on, if it was possible that "thousands of Russian regular troops will penetrate Ukrainian territory in the east of my country in July and August," I would have said, "no, this is not possible."
Read alsoPoroshenko: EU deal gradually compensating for Ukraine's losses caused by RussiaWith those moves, Poroshenko said, the world "is completely changed."
"Russian aggression completely destroyed the post-war global security system," the Ukrainian president declared.
Twenty-five years after independence, he said, the fight goes on – "fighting for freedom, fighting for democracy, fighting for sovereignty and territorial integrity."
