Imprisoned Nobel Peace Prize laureate Narges Mohammadi underwent a complex surgery in Iran that saw part of a bone in her right leg removed over cancer fears but was immediately returned to prison, raising the risks to her life, rights groups warned.

A letter signed by over 40 activist groups, sent to the United Nations Human Rights Council, urged that Mohammadi be immediately released on a medical furlough from a prison sentence on charges long criticised internationally. It is part of a wider pressure campaign on Iran over Mohammadi's detention since the Nobel committee honoured her last year.

Meanwhile, a group says another activist set himself on fire in prison this past weekend to protest against his incarceration as Iran continues to face internal dissent after years of protests against its theocracy.

“We urge that Iranian authorities stop the criminalisation of human rights, and refrain from summoning human rights defenders, journalists and writers to serve their prison sentences while their health situation is precarious,” said the letter, dated Monday.

Iran's mission to the United Nations did not immediately respond to a request for comment Tuesday. Iranian state media, which broadly ignored previous campaigns seeking to free Mohammadi, did not immediately report on the letter.

Mohammadi, 52, is serving sentences totaling 13 years and nine months in prison on charges of collusion against state security and propaganda against Iran's government. She has kept up her activism despite numerous arrests by Iranian authorities and spending years behind bars. That includes backing the nationwide, women-led protests sparked by the 2022 death of Mahsa Amini.

She suffered multiple heart attacks while imprisoned before undergoing emergency surgery in 2022, the letter says. In November of this year, her lawyer announced that doctors found a bone lesion that they feared could be cancerous, sparking the surgery she underwent on Thursday.

“She was transferred back to prison after only two days, against her doctor's advice and another request from her legal team that she be granted a medical furlough and sentence suspension,” the letter said.

“Years of imprisonment and months of solitary confinement have severely compromised Mohammadi's health, leaving her with multiple serious conditions that cannot be addressed through a short, incomplete hospital visit.”

Iran's economy has been in tatters for years because of sanctions imposed by the West. Its people are angry over the devaluation of their money and government corruption. That's fuelled protests, as well as a government crackdown on dissent in the country. The return of President-elect Donald Trump to the White House this January has stoked concerns for some that he may resume his “maximum pressure” campaign on the Islamic Republic.

Concerns are rising for other imprisoned activists in Iran as well. On Saturday, Saeid Gharibi set himself on fire to protest his 15-year sentence and conditions at Shiraz's Adelabad Prison, the New York-based Centre for Human Rights in Iran said. The centre quoted his lawyer as saying Gharibi suffered severe burns to his back and hands and has been denied medical care. Iran has not acknowledged the incident.

Last week, a former journalist with the Voice of America's Farsi service jumped to his death from a building in Tehran in protest of the country's supreme leader and an ongoing crackdown on dissent.