BBC: US President Barack Obama has condemned Russian "aggression" in Ukraine.
Speaking in Warsaw to mark 25 years since the fall of communism in Poland, he hailed Polish democracy as a beacon for neighbouring Ukraine.
"How can we allow the dark tactics of the 20th Century to define the 21st?" he said.
Earlier Mr Obama met Ukraine President-elect Petro Poroshenko, and pledged support for plans to restore peace to the country.
Mr Obama called Mr Poroshenko a "wise selection" to lead Ukraine, and said the nation could become a vibrant, thriving democracy if the world community stood behind it. (+ video) » | Wednesday, June 04, 2014