Washington DC - President Joe Biden directly blamed Russian leader Vladimir Putin on Friday for the reported death of Alexei Navalny in prison, describing the Kremlin critic as a "powerful voice for the truth."

Saying he was "outraged," Biden said he did not yet know exactly what had happened to 47-year-old Navalny but that it was the fault of Putin and his "thugs."

"Make no mistake, Putin is responsible for Navalny's death," Biden said in televised remarks from the White House. "What has happened to Navalny is yet more proof of Putin's brutality. No one should be fooled."

Biden, who has branded Putin a dictator over his invasion of Ukraine and treatment of opponents, said he was "looking at a whole number of options" on how to respond but did not go further.

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Asked in 2021 after meeting Putin in Switzerland what would happen were Navalny to die, Biden said he told the Russian leader that Moscow would face "devastating" consequences.

On Friday, Biden hailed Navalny's achievements and sent condolences to the Kremlin opponent's wife and children.

Biden recalled that Navalny had been poisoned while in exile and had then returned to Russia despite knowing the risks to his safety. Navalny was "so many things that Putin is not," he said.

"Even in prison, he was a powerful voice for the truth," Biden said. "God bless Alexei Navalny; his courage will not be forgotten."

Russian officials said Navalny died on Friday in an Arctic prison, a month before an election poised to extend Putin's hold on power.

Navalny's death after three years in detention and a poisoning that he blamed on the Kremlin deprived Russia's opposition of its figurehead at a time of intense repression and the invasion of Ukraine.

"We don't know exactly what happened," Biden said, "but there is no doubt that the death of Navalny was the consequences of something that Putin and his thugs did."

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Democrat Biden said Navalny's death meanwhile showed the need for Republicans in the US House to stop blocking a multi-billion-dollar war aid package for faltering Ukraine.

"This tragedy reminds us of the stakes of this moment," he said. "We have to provide the funding so Ukraine can keep defending itself against Putin's vicious onslaught and war crimes."

He played down concerns, however, about a Russian anti-satellite weapon that the White House this week said Moscow was developing, saying it posed "no nuclear threat" to the United States or any other country.

Biden also pledged that Washington would honor its "sacred commitment" to NATO, as allies worry that Republican Donald Trump could stage a comeback in November's presidential election.

He repeated his condemnation of "dangerous" and "outrageous" remarks by former president Trump encouraging Russia to attack NATO countries that don't meet their financial commitments.

"From Truman on, they're rolling over in their graves hearing this," Biden said, referring to President Harry Truman, who led the United States to become a founding member of the alliance in 1949.