Jose Diaz, spokesman for UN high commissioner for human rights, Louise Arbour, said officials at the global body were monitoring the case of Ogulsapar Muradova.
The International Helsinki Federation for Human Rights announced that Muradova's body had been seen by her relatives Thursday. The group's director blamed the government for what he said appeared to have been her violent death.
At the UN's European headquarters in Geneva, Diaz said: "We are very disturbed about the death in a Turkmen prison of Ogulsapar Muradova.
"We urge the Turkmen authorities to conduct a thorough, prompt and independent investigation into the cause of her death, including an independent medical examination of the body, and to make public the results of that inquiry."
Muradova was affiliated with the Turkmenistan Helsinki Foundation, and was a reporter with US-funded Radio Liberty. She and two other rights activists were arrested in June and later sentenced ranging from six to seven years, according to the International Helsinki Foundation.
Diaz said authorities had charged her with "illegal arms possession after a trial widely reported to be unfair." He called her "a human rights defender."
The human rights group, Diaz said, was concerned about the fate of the Amankurban Amanklychev and Sapardurdy Khajiyev, two other journalists who were arrested with Muradova.
The media freedom advocacy group, Reporters Without Borders, also has demanded a full investigation into Muradova's death.
Another media rights group expressed dismay with how Turkmen authorities "have not yet made public the time and cause of death."
"Their secretive conduct, combined with unofficial accounts of wounds found on her body, raise suspicions of foul play," said Joel Simon, executive director of the Committee to Protect Journalists.