Donald Trump, who drew criticism as US president for his praise of Russian leader Vladimir Putin, made his first public comment on the death of Russian opposition leader Alexei Navalny on Monday in a social media post that cast no blame.

"The sudden death of Alexei Navalny has made me more and more aware of what is happening in our Country," Trump wrote, appearing to link the death to his own political troubles.

"It is a slow, steady progression, with CROOKED, Radical Left Politicians, Prosecutors, and Judges leading us down a path to destruction. Open Borders, Rigged Elections, and Grossly Unfair Courtroom Decisions are DESTROYING AMERICA. WE ARE A NATION IN DECLINE, A FAILING NATION! MAGA2024"

It was not clear what similarities Trump was trying to draw with Russia's most prominent opposition leader. Navalny, 47, fought for years against what he called vast corruption in Putin's Russia, ruled by "crooks and thieves."

The Trump campaign did not immediately respond to a request for clarification.

Trump has railed against a judge's order on Friday to pay US$355 million in penalties for overstating his net worth to dupe lenders, a decision he called politically motivated. Trump also is preparing for four upcoming criminal trials as he pursues the Republican nomination.

President Joe Biden on Friday directly blamed Putin for Navalny's death in a the penal colony north of the Arctic Circle, as did Trump's main Republican rival, Nikki Haley. "Putin is responsible for Navalny's death," Biden said.

The Kremlin has denied involvement in his death and said that Western claims that Putin was responsible were unacceptable.

Since Navalny's death was reported on Friday, former US presidents and top members of Congress from both parties also denounced Putin.

But Trump, the frontrunner for the Republican nomination to challenge Biden in the November election, had remained silent until Monday.

During his 2017-2021 White House tenure, Trump expressed admiration for Putin. In 2018, he refused to blame the Russian leader for meddling in the 2016 US election, casting doubt on the findings of his own intelligence agencies and sparking criticism at home.

Last week, he suggested the United States might not protect Nato allies who aren't spending enough on defence from a potential Russian invasion.

Haley, the former South Carolina governor who will face Trump as an underdog in her home state's presidential primary on Saturday, called his response on Monday unpatriotic.

"Donald Trump could have condemned Vladimir Putin for being a murderous thug. Trump could have praised Navalny’s courage," she wrote on X. Instead, she said, he denounced America and compared it to Russia.

Republican former US Representative Liz Cheney, a vice chair of the congressional panel that investigated the January 6, 2021, storming of the Capitol by Trump supporters, recalled Trump's frequent promise to seek "retribution" against political opponents if he regains power.

"What Vladimir Putin did to Navalny is what retribution looks like in a country where the leader is not subject to the rule of law," Cheney said on Sunday. (Reuters)