Reports of Navalny's death shook the annual Munich Security Conference where leaders gathered to bolster unity against Russia's two-year-old invasion of Ukraine, the Reuters news agency reported.
"Let us be clear, Russia is responsible," Harris said, adding that it would be "a further sign of Putin's brutality" if the death is confirmed.
US Secretary of State Antony told the conference that if reports of Navalny's death are true, they testify to Russia's "weakness and rot."
Russian authorities said Navalny died after a fall at an Arctic penal colony on Friday.
US President Joe Biden had warned Putin after a meeting with the Russian leader in Geneva in June 2021 that Russia would risk "devastating" consequences if Navalny died in prison, Reuters reported.
Russia invaded Ukraine on February 24, 2022, starting the largest armed conflict in Europe since World War II.
Friday is day 723 of Russia’s war on Ukraine.
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Source: Reuters, PAP, IAR