Russian opposition activist Alexei Navalny, a prominent critic of President Vladimir Putin, died Friday at a jail in the Arctic region, Tass and Interfax news agencies said, a development that prompted Western leaders to issue harsh criticism of the country's authorities under Putin.

Quoting the Federal Penitentiary Service, Tass reported that Navalny, who was serving a sentence in northern Russia's Yamalo-Nenets Autonomous Region over charges of fraud and extremism, "felt unwell" after a walk and "almost immediately" lost consciousness.

Medics arrived right away and attempted to resuscitate him, but failed, the report said.

File photo shows Alexei Navalny giving a speech in Moscow in July 2019. (AP/Kyodo)

Tass quoted Kremlin spokesman Dmitry Peskov as saying that Putin received a report on Navalny's death.

In Washington, U.S. President Joe Biden told reporters that he was "outraged" by reports of the death of the 47-year-old activist. "Make no mistake, Putin is responsible for Navalny's death," he said.

Biden criticized Putin for having Navalny "poisoned," "arrested" and "prosecuted for fabricated crimes."

"If confirmed, this would be a further sign of Putin's brutality," U.S. Vice President Kamala Harris said in an address at a security conference in Munich, Germany.

Charles Michel, president of the European Union, said the bloc "holds the Russian regime" solely "responsible for this tragic death" in a post on the X social media platform.

Alexei Navalny is shown in a photo posted to his Instagram account on Sept. 19, 2020. (Kyodo)

"Navalny fought for the values of freedom and democracy. For his ideals, he made the ultimate sacrifice," Michel said.

The report of Navalny's death comes as Russia's invasion of Ukraine is about to enter its third year. Ukrainian President Volodymyr Zelenskyy said Navalny was "obviously" killed by Putin during a press conference in Germany with Chancellor Olaf Scholz.

"This is terrible news," British Prime Minister Rishi Sunak said on X. "As the fiercest advocate for Russian democracy, Alexei Navalny demonstrated incredible courage throughout his life," he said.

Peskov reacted sharply to such statements, saying the Kremlin considers them totally unacceptable as "there has been no information regarding the cause of death," according to Tass.

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