China's longest-serving female political prisoner, a Tibetan nun, was exiled from China and arrived in the United States on Friday.

Ngawang Sangdrol, 26, the leader of a group of Tibetan independence activists known as the "singing nuns," was accompanied on the plane by a U.S. diplomat and traveled to Chicago. She was released on medical parole in October after negotiations with U.S. officials and human rights activists.

Sangdrol had been serving a 21-year sentence and was scheduled for release on Nov. 3, 2011.

She was first detained at age 13 for opposing China's rule in Tibet and, according to human rights reports, was badly beaten during her nine-month detention. She was barred from rejoining her convent after release. She was arrested again in 1992 at age 15 and sentenced to three years for attempting to demonstrate.

She rose to prominence after a tape of songs she made with 13 other imprisoned Buddhist nuns was smuggled out of Tibet's infamous Drapchi prison. The songs spoke of the nuns' love for their families and Tibet. Sangdrol's sentence was extended by six years after authorities learned of the tape. She was reportedly beaten again.

In 1996, she conducted protests in prison and was sentenced to another four years. In 1998, she was given a third extension for prison protests, including one linked to the visit of a delegation of European Union ambassadors. She became a hero to many Tibetans in China and overseas.

Sangdrol's departure from China marks another in a series of moves by Beijing to improve ties with the United States.

She is the fourth prominent Tibetan political prisoner to be released since 2001. Xu Wenli, the leader of the banned China Democracy Party, was paroled in December and left for the United States.

Like Xu, Sangdrol was paroled on medical grounds. She was allowed to leave China to seek treatment for severe headaches, which Chinese doctors were unable to cure, said John Kamm, the president of the Dui Hua Foundation, an organization that lobbies on behalf of Chinese political prisoners.

Sangdrol's release came after years of campaigning by Tibetan and human rights organizations and follows sustained efforts to secure her departure by the United States, France and other governments.

Assistant Secretary for Human Rights Lorne Craner recently returned from Beijing, where details were apparently finalized. Lodi Gyari, a special envoy of the Dalai Lama, specifically urged that she be given medical parole when he visited Lhasa and Beijing last September.

Beijing claims that Tibet has been part of China since the 13th century. Many Tibetans say that Tibet should be an independent country.

The Dalai Lama fled Tibet in 1959 following an aborted uprising by Tibetans against Chinese rule. He has since announced that he has abandoned his dreams of an independent Tibet and is willing to acknowledge Chinese sovereignty over his homeland. China says the Dalai Lama is not sincere.

Ngawang Sangdrol was first arrested at age 13 and had her prison sentence extended for leading protests. She is now in the United States for medical treatment.