Gao, 69, produced works which criticised former leader Mao Zedong.

"We are concerned by the trial of Gao Zhen, a prominent contemporary artist, on charges of 'slandering heroes and martyrs' relating to satirical sculptures he created more than a decade before the offence under which he is charged was introduced into law," UN Human Rights Office spokesman Seif Magango said in a statement.

"The case against him raises concerns with regard to retroactive application of criminal law and use of criminal sanctions to punish artistic expression, undermining the principle of legality."

Gao Zhen, who emigrated to the United States in 2022, was detained at his studio on the outskirts of Beijing in August 2024 during a visit to China.

The UN rights office is concerned by reports that Gao's health is deteriorating in detention and urged the authorities to release him immediately and unconditionally.

Magango said his trial, held behind closed doors, concluded on March 30 and the artist remains behind bars, pending judgement.

His wife and their seven-year-old son, a US citizen, have been barred from leaving China, with the UN rights office calling for the exit ban to be lifted.

Gao and his brother Gao Qiang gained fame in the early 2000s for art touching on sensitive political topics like the Cultural Revolution, the Tiananmen Square protests and the legacy of Mao, who led China from 1949 until his death in 1976.

Magango also noted the release of high-profile human rights lawyer Yu Wensheng on April 13 following a 36-month sentence on charges of "inciting subversion of state power".

"It is crucial that his release is unconditional, that any related restrictions on him and his wife Xu Yan, including on freedom of movement and expression, are lifted, and that they are free from surveillance," he said.

"We also urge the Chinese authorities to immediately and unconditionally release all other individuals currently detained for peacefully exercising their rights, including under the vague and overly broad provisions related to 'inciting subversion' and 'picking quarrels and provoking trouble'," the spokesman added.