"It is absolutely the same situation like Russian bombardment in Aleppo," he told CNN's Christiane Amanpour in an exclusive interview Wednesday.
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 the 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 on Ukrainian territory in the east of my country in July and August," I would have said, "no, this is not possible."
With those moves, he said, the world "is completely changed."
"Russian aggression completely destroyed the post-war global security system," he said.
Twenty-five years after independence, he said, the fight goes on -- "fighting for freedom, fighting for democracy, fighting for sovereignty and territorial integrity."
America a 'reliable partner'
Ukraine has become a somewhat surprisingly prominent issue in the American election.
That's due in part to Donald Trump's former campaign chairman, Paul Manafort, who has served as an adviser to former Ukrainian President Viktor Yanukovych, who fled office in 2014 after massive protests in the country.
It was recently announced
that Ukraine's anti-corruption bureau is investigating Manafort and whether he received millions in potentially illegal cash payments from the country's former ruling party. The announcement of the investigation followed an August 14 report published by The New York Times
, which first revealed that Ukrainian investigators had found Manafort's name included in an off-the-books, handwritten ledger detailing a series of secret payments -- among them, a $12.7 million payout to Manafort.
He has denied the details in the report, saying, "I have never received a single 'off-the-books cash payment' as falsely 'reported' by The New York Times, nor have I never done work for the governments of Ukraine or Russia."
Poroshenko would not comment on the specifics of the case, but pledged that any possible charges -- against Manafort or anyone else -- would only come after a "transparent and independent investigation."
Trump, too, has stirred controversy with his statements on Russia. The Republican presidential nominee has been criticized for his comments expressing openness to recognizing Russia's annexation of Crimea, and has called on the Russian government to share emails it possibly hacked from Democratic nominee Hillary Clinton and drawn rebukes from critics who say he's soft on a traditional US adversary.
"I have no doubt that we will have bipartisan support after the presidential election," Poroshenko said. "And I count on the responsible choice of the American people, no matter it would be Republican or Democrat."