Russian authorities have violated the rights of jailed opposition leader Alexey Navalny by restricting his contact with lawyers and repeatedly subjecting him to solitary confinement, the United States has said.

Navalny – who was Russian President Vladimir Putin’s most vocal domestic critic before his arrest – is serving nine years in jail on charges he says are politically motivated.

“The United States is deeply concerned by the Russian Government’s escalating, arbitrary interference with Aleksey Navalny’s rights,” the US State Department said in a statement late on Friday.

The US agency said that Russian prison authorities had interfered with Navalny’s access to legal defence by supervising his meetings with his lawyers and delaying exchanges of documents and communication between them.

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

In recent weeks, the 46-year-old opposition leader who is jailed near the town of Vladimir some 200 kilometres (125 miles) outside Moscow, had said he was placed in solitary confinement on several occasions as punishment.

On Wednesday, Navalny posted on Twitter via his lawyers that he had been ordered to spend 15 more days in solitary confinement, which he said was the fourth time he had been placed in isolation at the prison.

Describing Navalny’s arrest as “already shameful”, the State Department said Moscow’s “insistence on harassing him further only highlights its insecurity and fear of those who speak the truth”.

In mid-August, Navalny issued a call from his prison cell for the West to impose tougher sanctions on Russian oligarchs.

Images of the opposition leader, published last week by independent Russian media, showed him emaciated and exhausted. His aides have also raised an alarm, saying that new communication restrictions at the prison prevent them from knowing what is happening to him.

Navalny miraculously survived a poisoning attempt on a domestic flight in Siberia in August 2020, after which he was airlifted to Germany for months of medical treatment before voluntarily returning to Russia.

On his return, he was immediately arrested for breaking the terms of parole that he was on. (Al Jazeera)

•PHOTO: Opposition leader Alexey Navalny was sentenced to nine years in a maximum-security penal colony. [File: Moscow City Court/AP]

Update news