DUBAI, United Arab Emirates (AP) — 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 criticized internationally. It is part of a wider pressure campaign on Iran over Mohammadi's detention since the Nobel committee honored 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 criminalization 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 Ir.