WARSAW (AFP) - US President Barack Obama on Wednesday condemned the "dark tactics" and "aggression" of Russia in Ukraine after meeting Kiev's president-elect Petro Poroshenko and promising him years of US support.

Adopting the mantle of 'leader of the West' Obama hailed Poland for building a thriving democracy 25 years after elections which fractured communism as part of a European tour dedicated to bolstering Nato defenses and resolve.

He said the country's long struggle against occupation and tyranny was mirrored in the plight of Ukraine a quarter century later which showed freedom was still under threat.

"We will not accept Russia's occupation of Crimea or its violations of Ukraine's sovereignty," Obama said, just over an hour after meeting Poroshenko in Warsaw, ahead of his own expected encounter with President Vladimir Putin in France on Friday.

"Our free nations will stand united so that further Russian provocations will only mean more isolation and costs for Russia.

"Because after investing so much blood and treasure to bring Europe together, how can we allow the dark tactics of the 20th century to define this new century.

"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."

"We have to work for that. We have to stand with those who seek freedom."

The US leader, forced to renew US reassurances to former Soviet states in Nato amid the worst East-West crisis since the end of the Cold War, also warned that freedom must not be taken for granted.

"The story of this nation reminds us that freedom is not guaranteed," Obama said.

"And history cautions us to never take progress for granted.

"On the same day 25 years ago that Poles were voting here, tanks were crushing peaceful democracy protests in Tiananmen Square on the other side of the world.

"The blessings of liberty must be earned and renewed by every generation -including our own. This is the work to which we rededicate ourselves today."

Obama also offered a robust security guarantee to fellow Nato members who feel threatened by Russian moves in Ukraine.

He reaffirmed that Article Five of the Nato charter would oblige the alliance to come to the defence of any state that was attacked.

"Poland will never stand alone," Obama said.

"Estonia will never stand alone. Latvia will never stand alone. Lithuania will never stand alone. Romania will never stand alone."