The U.S. State Department released a statement on Friday expressing concern about the Russian government’s “interference” with the rights of imprisoned Russian opposition leader Alexey Navalny, an outspoken critic of President Vladimir Putin.

“The United States is deeply concerned by the Russian Government’s escalating, arbitrary interference with Aleksey Navalny’s rights,” wrote department spokesperson Ned Price.

Price said that prison officials have interfered with Navalny’s defense by forcing supervision of meetings with his lawyers and imposing delays on the exchange of documents between Navalny and his counsel.

“This interference, along with his repeated diversion to solitary confinement for minor alleged infractions, is further evidence of politically motivated harassment,” Price said.

“His arrest upon return to Russia was already shameful, but the Russian Government’s insistence on harassing him further only highlights its insecurity and fear of those who speak the truth.”

The U.S. has said previously that Navalny was poisoned with a nerve agent in an alleged assassination attempt by Russian officials in 2020 after he expressed support for the protests occurring in Belarus and predicted that similar turmoil would occur in Russia.

He was transferred to Germany for medical treatment following the poisoning.

The opposition figure was arrested upon his return to Russia the following January and sentenced to 2 1/2 years in prison.

The State Department released a statement last month condemning the attempt against Navalny on its two-year anniversary and calling for the Kremlin to “fully … dismantle its chemical weapons program.”

Since his initial arrest, Navalny’s sentence has been extended to more than 20 years on charges of fraud and contempt of court, counts that are contested by Secretary of State Antony Blinken and European Foreign Policy Chief Josep Borrell, among others.

The Russian government has also charged Navalny for creating an “extremist group in order to incite hatred towards officials and oligarchs.”