TALLINN, Estonia (AP) — Experts from the U.N. Human Rights Commission appealed to Belarusian authorities on Monday to pardon an opposition politician diagnosed with a severe form of cancer while in prison.

Ryhor Kastusiou, 66, the leader of the Belarusian Popular Front, or BPF, party, is serving a 10-year sentence on charges of “conspiracy to seize power.” The BPF is among Belarus’s oldest political parties, and was liquidated by the Belarusian Supreme Court in 2023.

The U.N. experts urged Belarusian authorities to “consider granting him a pardon or remission of sentence,” in view of Kastusiou’s “grave health condition.”

Kastusiou was detained in April 2021, and during his almost three years of detention, has been placed many times in punishment cells, known for their inhumane conditions.

He was reportedly diagnosed with cancer shortly after his arrest, but didn't receive his medical results in a timely manner, which “deprived him of the opportunity to effectively seek release from detention on medical grounds.”

The U.N. experts believe the poor conditions Kastusiou has endured, together with a failure by prison medical services to provide timely medical treatment and care, “may even amount to torture.” They say they are in contact with Belarusian authorities regarding Kastusiou’s case.

Opposition leader Sviatlana Tsikhanouskaya, in exile in neighboring Lithuania after challenging authoritarian Belarusian President Alexander Lukashenko in the 2020 presidential election, called for Kastusiou’s release.

“How many political prisoners must die in Belarusian prisons? Despite suffering from cancer, Ryhor Kastusiou has repeatedly been sent to a punishment cell and denied proper medical care,” Tsikhanouskaya said. “He must be released.”

Lukashenko unleashed an unrelenting crackdown on dissent in August 2020, after an election that the opposition and the West denounced as a sham gave him his sixth term in office.

The outcome of the vote triggered unprecedented mass protests that rocked the country for months. Belarusian authorities detained more than 35,000 demonstrators, with police and security forces brutally beating many. Thousands have fled the country, and dozens have been labeled extremists by authorities.

There are 1,421 political prisoners behind bars in Belarus, according to human rights group Viasna, including Nobel Peace Prize laureate Ales Bialiatski. According to Viasna, at least five political prisoners in Belarus have died behind bars.