PRESIDENT Barack Obama has met Ukraine's president-elect Petro Poroshenko, in a show of US support for Ukraine's right to chart its own future, before an encounter with Russia's Vladimir Putin.

Obama sat down with Poroshenko on Wednesday in Warsaw, during a trip designed to assuage security concerns in eastern Europe following Russia's annexation of Crimea and what Washington says is an effort to destabilise Ukraine.

Obama said he had "been deeply impressed" by Poroshenko's vision for his troubled country.

"The United States is absolutely committed to standing behind the Ukrainian people not just in the coming days, weeks, but in the coming years," Obama told reporters.

The talks on day two of Obama's European tour come after the president met central and eastern European leaders in Warsaw and before he heads to a G7 summit in Belgium.

The summit takes place against a backdrop of signs that Western unity over how to handle Russia is fracturing.

Obama will come face to face with Putin during 70th anniversary commemorations of the D-Day landings in Normandy, France on Friday, but officials in Washington and Moscow say there are no plans for a formal meeting.

In contrast, the leaders of Britain, France and Germany will hold one-on-one talks with Putin, who said Wednesday he could not understand Obama's stance.

"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.

Putin went on to accuse the US administration of hypocrisy in its "aggressive" attempts to isolate Russia over its conduct in 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."

The accelerating diplomacy over Ukraine comes as a seven-week pro-Russian insurgency in Ukraine's eastern rust belt grows only more violent after Poroshenko swept to power in a May 25 presidential ballot.

Hundreds of separatist gunmen on Monday attacked a Ukrainian border guard service camp in the region of Lugansk on the border with Russia.

Obama said Tuesday that US commitment to eastern European security was absolute.

"Our commitment to Poland's security as well as the security of our allies in central and eastern Europe is a cornerstone of our own security and it is sacrosanct," Obama said after inspecting a joint unit of Polish and US F-16 pilots.

He proposed a "European Reassurance Initiative" of up to $1 billion (730 million euros) to finance extra US troop and military deployments to "new allies" in Europe.

NATO defence ministers also agreed Tuesday a series of steps to bolster protection in eastern Europe after the Ukraine crisis, but insisted they were acting within the limits of a key post-Cold War treaty with Moscow.