Imprisoned Russian opposition politician Alexei Navalny is dead. Seen as President Vladimir Putin’s most vociferous critic, Navalny was serving a 19-year jail term for offences widely considered politically motivated, reports the BBC.

He was moved to an Arctic penal colony, considered one of the toughest jails, late last year. The prison service in the Yamalo-Nenets district said he had “felt unwell” after a walk on Friday.

He had “almost immediately lost consciousness”, it said in a statement. The causes of his death were being established, Tass news agency reported.

The White House said the death in prison of Russian opposition leader Alexy Navalny, if confirmed, would be “a terrible tragedy.”

Speaking on NPR, White House National Security Advisor Jake Sullivan also added the Kremlin’s “long and sordid” history of harming its opponents “raises real and obvious questions about what happened here.”

Ukrainian President Volodymyr Zelenskyy said Russian leader Vladimir Putin must be held accountable for the death of opposition leader Alexey Navalny, following reports of his death.

‘Alexey Navalny died in a Russian prison — it is obvious for me that he was killed,” Zelenskyy told reporters at a joint news conference in Berlin with German chancellor Olaf Scholz, reports CNN.

”Putin does not care who dies as long as he stays in his position – and this is why he should lose everything. This is why he should lose the war and he should be held accountable for the crimes that were committed on his behalf,” Zelensky went on to say.

”Like thousands of others who have been tortured to death because of this single Russian leader,” Zelensky said.

Agnes Callamard, Secretary General, Amnesty International, said , “They killed him. I am heartbroken. I am thinking of his wife and kids, his friends and colleagues. RIP courageous #AlexeiNavalny. May you find many wonderful rivers to swim in.”