Story highlights On her visit, Ambassador Samantha Power says U.S. support for Ukraine is "unwavering"

Power urges Ukrainians to demand change, but she acknowledges their suffering

(CNN) A senior U.S. diplomat, on a visit Thursday to Ukraine, has slammed what she called Russian "aggression" in the country.

Samantha Power, the U.S. ambassador to the United Nations, pledged support to Ukraine and delivered a pep talk of sorts that encouraged Ukrainians to "demand change."

"America is clear-eyed when it comes to seeing the truth about Russia's destabilizing" actions in Ukraine, she said, adding that U.S. support for Ukrainians is "unwavering."

If Russia "continues to violate the rules upon which international peace and security" stands," Power said, then the United States will "raise the costs on Russia" and "rally other countries to do the same."

"Their silence in the face of Russian aggression will not placate Moscow," she said. "It will only embolden it."

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