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.
Mr Poroshenko, a billionaire sweet manufacturer, was elected in May.
Mr Obama pledged $5m (£3m) of military assistance to Kiev including body armour and night-vision goggles.
The aid follows $18m promised since early March for food, clothes, radios and other equipment.
Mr Obama is now in Brussels for a meeting of the G7 major industrial nations, the first since Russia was removed from the G8 in protest over its annexation of Crimea in March.
He is expected to attend the 70th anniversary commemorations of the D-Day landings in Normandy on Friday. Russian President Vladimir Putin will also be at the ceremony but the two leaders have no meeting scheduled.
Mr Poroshenko will be at the commemorations ahead of his inauguration on Saturday. He said he did not rule out meeting Mr Putin.
Later, when asked who he was prepared to meet at the ceremony, Mr Putin said he was "not going to avoid any of them".
In an interview with French TV, he ridiculed US claims it had evidence of Russian military interference in eastern Ukraine.
"The entire world remembers the US secretary of state demonstrating the evidence of Iraq's weapons of mass destruction, waving around some test tube with washing powder in the UN Security Council," he said.
The BBC's Adam Easton in Warsaw says Mr Obama chose to come to Poland for its symbolic value.
In his speech, the US president said that 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."
Later German Chancellor Angela Merkel threatened Russia with further sanctions if Moscow failed to rein in separatists in eastern Ukraine.
"It is decisive that President Putin use his influence to get the separatists to refrain from violence and intimidation, hand over their weapons and stop the occupations," she said in a speech to parliament.
Meanwhile, on the ground in Ukraine, separatist rebels have taken two military bases in the eastern region of Luhansk.
Separatists seized a border guard base after days of fierce combat, and a National Guard base after an attack which began on Tuesday.
Fighting continues near the towns of Krasny Liman and Sloviansk in neighbouring Donetsk region.
Ukrainian sources say rebels are trying to break out of encirclements by government forces.