Top Kremlin critic Alexei Navalny says he's a 'skeleton walking, swaying, in its cell'

• Alexei Navalny says he looks like a "skeleton" amid reports his health is rapidly deteriorating.
• The imprisoned Kremlin critic has been on a weekslong hunger strike.
• Navalny, who was poisoned in August, is demanding access to his own doctors.

Imprisoned Kremlin critic Alexei Navalny joked about the state of his health in an Instagram post on Tuesday, three weeks into a hunger strike over a demand for proper medical care.

"If you saw me now, you would laugh," Navalny said, describing himself as "a skeleton walking, swaying, in its cell." The Russian opposition leader has been posting on Instagram from prison via his lawyers.

Navalny's allies have warned he's on the verge of death and leaders across the world have condemned the Russian government over his imprisonment. Thousands of Russians have demonstrated against Navalny's detention and demanded his release.

The anti-corruption campaigner, who is Russian President Vladimir Putin's most prominent critic, was poisoned with the Soviet-era nerve agent Novichok in August. Navalny has blamed the incident on Putin, whose opponents often wind up dead in violent or suspicious ways.

After receiving treatment in Germany for several months following his poisoning, Navalny returned to Moscow in January and was promptly arrested. He was charged with violating the terms of a suspended sentence for fraud in 2014, and sentenced to two and a half years in prison. Navalny has dismissed all charges against him as politically motivated.

While in prison, Navalny complained of acute back pain and numbness in one leg and demanded access to his own doctors. Earlier this week, Navalny was transferred to an infirmary at a separate penal colony as his condition worsened. Authorities in Russia continue to prevent Navalny's doctors from seeing him in prison.

The White House has warned that there will be consequences for Moscow if Navalny dies. But Navalny's chief of staff is urging the US to do more.

"I prefer that Putin be held accountable for what's happening now, before Navalny dies. I don't want my friend and my colleague to die," Leonid Volkov told CNN.

Meanwhile, UN experts on the Human Rights Council on Wednesday warned that Navalny is in "serious danger" and called for his "urgent medical evacuation from Russia."

"We are deeply troubled that Mr. Navalny is being kept in conditions that could amount to torture or cruel, inhuman or degrading treatment or punishment in a facility that reportedly does not meet international standards,"the experts said in a statement.