Obama met Ukraine's president-elect Petro Poroshenko in Warsaw and promised years of US support, then blasted Russia and vowed to protect ex-Soviet states in NATO in a hawkish speech marking 25 years of Polish democracy.
"How can we allow the dark tactics of the 20th century to define this new century?" Obama asked.
"As we've been reminded by Russia's aggression in Ukraine, our free nations cannot be complacent in pursuit of the vision we share - a Europe that is whole and free and at peace."
Obama will come face-to-face with Putin on Friday in France.
Though several European leaders are meeting the Russian leader and hope to pursue dialogue to ease the Ukraine crisis - which saw hundreds of rebels battle government forces on Wednesday - Washington remains to be convinced.
"We will not accept Russia's occupation of Crimea or its violations of Ukraine's sovereignty," Obama said, as Defense Secretary Chuck Hagel announced in Brussels a review of "the US force presence in Europe" amid the Ukraine crisis.
Putin said he could not understand why Obama, who has spent months trying to isolate him over Ukraine, would not hold a formal meeting with him during 70th anniversary commemorations of the Normandy landings in World War II.
"It is his choice, I am ready for dialogue," Putin said in an interview with French broadcasters Europe1 and TF1 conducted at his dacha in the Black Sea resort of Sochi.
The Russian leader accused the US of hypocrisy in its "aggressive" attempts to punish Russia over Ukraine.
"We have almost no military forces abroad yet look: everywhere in the world there are American military bases, American troops thousands of kilometres from their borders.
"They interfere in the interior affairs of this or that country.
So it is difficult to accuse us of abuses."
In Ukraine, three government soldiers have been injured in a massive all-night attack carried out by hundreds of pro-Russian insurgents in the nation's restive east.
The assault on a position held by the Ukrainian National Guard in the Lugansk region began on Tuesday evening and lasted 10 hours, the interior ministry said, adding that six rebels were killed in the fighting.
The 300 rebels who took part in the attack were armed with automatic weapons, rocket launchers and mortars, and the Ukrainian forces fought "to the last bullet," according to the ministry.
