The United States said Friday that the death in prison of Russian opposition leader Alexy Navalny, if confirmed, would be "a terrible tragedy."

White House National Security Advisor Jake Sullivan added the Kremlin's "long and sordid" history of harming its opponents "raises real and obvious questions about what happened here."

Navalny, 47, died Friday in an Arctic prison according to Russia's penitentiary service. He survived a poisoning attack in 2020 only to be sentenced to 19 years in a penal colony.

Sullivan told National Public Radio that the White House was "actively seeking confirmation" to decide "what comes next."

Navalny's press secretary Kira Yarmysh said his team had not been informed of his death and that his lawyer was on his way to the prison.

President Joe Biden made no immediate statement on the reports of Navalny's death.

Asked in 2021 after meeting President Vladimir Putin in Switzerland, what would happen were Navalny to die, Biden said: "I made it clear to him that I believe the consequences of that would be devastating for Russia."