US President Barack Obama has condemned Russian "aggression" in Ukraine, BBC reported Wednesday.
"How can we allow the dark tactics of the 20th Century to define the 21st," Obama said here.
While speaking in Poland's capital city of Warsaw to mark 25 years since the fall of communism in Poland, he hailed Polish democracy as a beacon for neighbouring Ukraine.
Earlier Obama met Ukraine President-elect Petro Poroshenko, and pledged support for plans to restore peace to the country.
The US president described Poroshenko a "wise selection" to lead Ukraine, and said the nation could become a vibrant, thriving democracy if the world community stood behind it.
Poroshenko, who was elected in May, will be inaugurated as Ukraine's president Saturday.
Obama also pledged five million dollars (about 3 million pounds) of military assistance to Kiev including body armour and night-vision goggles. The aid follows 18 million dollars promised since early March for food, clothes, radios and other equipment.