TALLINN, Estonia (AP) — The most prominent human rights group in Belarus is calling on the United Nations’ special rapporteur for human rights in the repressive country to pressure officials to give adequate medical treatment to an imprisoned opposition leader suffering from severe cancer.

The appeal Wednesday by the Viasna group said Ryhor Kostusiou is being denied proper medical care in the prison where he is serving a 10-year sentence on a conviction of attempting to unlawfully seize power.

Kostusiou, head of the Belarusian Popular Front party, was arrested in 2021 as part of a harsh crackdown on opposition that began after mass protests shook the country following the 2020 presidential election. The election’s disputed results gave another term to President Alexander Lukashenko, who has been in office since 1994 and suppressed opposition and independent news media.

About 35,000 people were detained in and after the protests. More than 1,400 political prisoners remain behind bars, including Viasna founder Ales Bialiatski, a winner of the Nobel Peace Prize in 2022.

Viasna said Monday that another political prisoner had died of pneumonia after authorities ignored his pleas for help.

The 67-year-old Kostusiou’s “health condition has seriously deteriorated,” Viasna said in its appeal to special rapporteur Anais Marin. ”The conditions for serving a criminal sentence do not take into account his serious health condition, do not allow him to receive quality treatment and endanger his life.”

The group asked the rapporteur to send “an urgent appeal to the authorities asking them to provide Kostusiou with proper treatment.”