Syrian state television has aired an interview with a woman who had reportedly been found decapitated, armless and skinned in a morgue last month, amid a deadly protest crackdown.
Zaynab al-Hosni became a symbol of the revolt against President Bashar al-Assad after international human rights watchdogs said late September that her mother had found her mutilated body in a morgue.
But a woman appeared on Syrian state television on Tuesday night, flashing her identity card as proof she was Hosni.
Amnesty International and Human Rights Watch (HRW), in a joint statement, on Wednesday called for an independent investigation to determine the true identity of the mutilated body.
"Zaynab al-Hosni's family subsequently confirmed that the woman who appeared on Syrian television is indeed Zaynab al-Hosni. The family has not been able to speak to her to verify her current situation," they said.
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"It now appears that Zaynab's family misidentified the body that was presented to them due to the extensive damage to the body."
Hosni said on state television: "I fled my family (in late July) because my brother beat me. My parents do not know where I am ...
"I learned about my own death on television channels which said the Syrian security forces detained me, and burned and cut up my body."
Amnesty and HRW had issued statements in September denouncing the murder of the 18-year-old, whose body they said had been discovered in a morgue by her mother.
At the time, Amnesty said her family had come across her body when they went to the morgue on September 13 to identify her activist brother Mohammed, who was also apparently arrested, tortured and killed.
"Zaynab had been decapitated, her arms cut off, and skin removed," the London-based watchdog said at the time.
The New York-based HRW had said Zaynab al-Hosni vanished in late July and that the Syrian authorities returned her "dismembered body" to her family on September 17.