Washington DC/ IBNS: US President Joe Biden Friday said he was "not surprised" but "outraged" over the news of Russian opposition leader Alexei Navalny's death.

"He bravely stood up to the corruption, the violence and all the bad things the Putin government was doing," Biden said at the White House of Russian President Vladimir Putin. "Putin is responsible for Navalny's death."

Russian opposition leader and President Putin's critic Alexei Navalny has died at the Arctic prison colony, media reports said on Friday. He was 47.

He was serving a 19-year term.

Alexey Navalny felt unwell in prison after a walk and lost consciousness. He received all necessary resuscitation measures, but they did not yield positive results, the Federal Ptnitentiary Service stated as quoted by Russia's Sputnik news agency.

The White House sought more information about Navalny's death at a Russian penal colony north of the Arctic Circle, where he was dispatched less than two months ago.

The development has put a further chill into already bitter US-Russian relations.

The 47-year-old Navalny had been a leading critic of Putin, and Biden had said after meeting Putin in Geneva in June 2021 that Nalvany's death would risk devastating consequences for Putin.

Biden and Putin remain deeply at odds over Russia's invasion of Ukraine two years ago, and Biden is urging Republican hardliners in the US Congress to support additional funding to pay for more weaponry for Ukraine's military.

Authorities are trying to figure out the cause of Navalny's death.

Commenting on the development, Kremlin spokesman Dmitry Peskov was quoted as saying by Sputnik it is up to medics to find out reasons behind Navalny's death.

“As far as we know, now, in accordance with all current rules, the Federal Penitentiary Service is engaged in all inspections, clarifications, and so on. That is, no instructions are required anymore, because there is a certain set of rules in this regard that the service is now guided by,” Peskov told reporters, adding that the news about Navalny’s death was “reported from Moscow.”

Speaking about the investigation into the death of the Russian leader, authority said in a statement as quoted by Sputnik: "The investigation office of the Russian Investigative Committee in the Yamalo-Nenets Autonomous Area has launched a procedural investigation into the death of Navalny A.A. ... In accordance with the procedure established by law, a set of investigative and operational measures is being carried out aimed at establishing all the circumstances of the incident.

The prominent opposition leader had garnered a massive follower base for his criticisms of the corruption in Putin's administration.

According to reports, he returned to Russia in 2021 from Germany where he underwent treatment after being allegedly poisoned.

Upon his return, Navalnvy was swiftly arrested on charges he dismissed as politically motivated, reported CNN.