Price said in a statement that the Russian prison authorities had interfered in the preparation of Navalny’s defense and communication with his lawyer. He also said Navalny was repeatedly placed in solitary confinement for alleged minor abuses.

On Thursday, Navalny said he could no longer confidentially contact his lawyer. All correspondence and consultation with his legal advisors are controlled by the prison administration. On Twitter, where Navalny posts through his lawyer, the Kremlin critic wrote that the authorities refuse to explain why this action was taken.

Navalny is the main political opponent of President Vladimir Putin in Russia. He is serving an 11.5-year prison sentence after being found guilty of breaking rules while on probation, fraud and contempt of court charges. The opposition leader insists that all charges against him were fabricated to stifle dissent and thwart his political ambitions.

Last year, Navalny returned to Russia after receiving treatment in Germany for poisoning with the nerve agent Novichok by suspected Russian intelligence in Siberia.