"She belongs back in Ukraine, working alongside her colleagues in the Rada (parliament) to build a better future for her country," said Samantha Power in a statement.
"We call on Russia to release her at once."
Savchenko delivered a final statement in her defence during her trial in a Russian court where she stands accused of killing two Russian journalists during shelling in eastern Ukraine.
Power recalled that she had twice met Savchenko's mother, Maria, who has expressed alarm over her daughter's worsening health.
The 34-year-old military helicopter pilot has been on a dry hunger strike for nearly a week.
"Savchenko - as well as all Ukrainians who are being held illegally by separatists and by Russia - should be free," said Power.
The US ambassador has been a vocal supporter of Ukraine at the UN Security Council, repeatedly accusing Russia of backing separatists in their campaign against the Kiev government.
Savchenko is seen in Ukraine as a symbol of resistance against the Kremlin, accused of fuelling the conflict that has claimed more than 9,000 lives since April 2014.
In September 2014, she was elected to Ukraine's parliament, the Rada, while behind bars in Russia.
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