US President Barack Obama has warned Russia against its "dark tactics" in Ukraine.
In a speech in Warsaw marking 25 years of Polish democracy, Mr Obama said the United States was committed to supporting the people of Ukraine.
"How can we allow the dark tactics of the 20th Century to define the 21st?" he said.
The US president said Poland's long struggle against occupation and tyranny was mirrored in the plight of Ukraine today.
"We will not accept Russia's occupation of Crimea or its violations of Ukraine's sovereignty," Mr Obama said.
"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."
Earlier Mr Obama met Ukraine President-elect Petro Poroshenko, and pledged support for plans to restore peace to the country, the BBC reports.
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.
The US president pledged $US5 million of military assistance to Kiev including body armour and night-vision goggles.
The aid follows $US18 million promised since early March for food, clothes, radios and other equipment.