The European Union, marking the second anniversary of the "politically motivated" sentencing of Russian opposition leader Aleksei Navalny, says it is "deeply disturbed" over reports of his treatment behind bars.

The 46-year-old Kremlin critic was handed a 2 1/2-year prison sentence for violating the terms of an earlier parole when he was medically evacuated in a coma after suffering a near-fatal poisoning in August 2020 that he blames on Russian security operatives acting at President Vladimir Putin's behest.

The original conviction is widely regarded as a trumped-up, politically motivated case. In March 2022, Navalny was handed a separate nine-year prison term on charges of contempt and embezzlement through fraud that he and his supporters have repeatedly rejected as politically motivated.

"The verdict came after A. Navalny's arbitrary arrest and imprisonment upon his return to Russia after receiving life-saving medical care in Berlin. He needed it after an attempted murder in Russia in August 2020 using a chemical-warfare nerve agent prohibited by the Chemical Weapons Convention, to which Russia is a state party," the EU's top diplomat, Josep Borrell, said in a statement to mark the second anniversary of the sentencing on February 2.

Concern has been growing in recent months over Navalny's health and his treatment in prison.

Navalny, who has been a persistent thorn in Putin's side by exposing corruption and organizing pro-democracy protests, said on social media on February 1 that he was being placed in a solitary confinement cell for six months.

That comes on top of spending more than 100 days in solitary in the past six months for what he and his supporters say are dubious reasons.

"The main torment of imprisonment is, of course, the inability to see the faces of your family, to talk to your loved ones. I haven't had any visits for 8 months and yesterday I was told that I'd be transferred to a cell-type facility for the maximum possible term of 6 months," he wrote on Twitter.

"Even maniacs and serial killers serving life sentences have the right for meetings, but I do not," he added.

"When such thing happens to you, you understand even more acutely how important it is to fight this unscrupulous government, how important it is to do at least something to free Russia from the yoke of these scoundrels from Russia and dispel the dope with which they have enveloped the heads of millions," he added.

Since January 1, dozens of lawmakers, lawyers, and physicians in Russia have urged Putin, the Prosecutor-General's Office, and the presidential Council for Human Rights to intervene on behalf of Navalny amid reports his health has dramatically worsened.

"Reports of repeated ill-treatment, physical and psychological torture by the Russian prison authorities against A. Navalny are deeply disturbing. The Russian authorities are responsible for his safety and health, and we call them to account," Borrell said in his statement.

He added that the EU "stands in solidarity" with Navalny and others, including ordinary Russian citizens, "who dare to speak the truth despite the great risk to themselves."