A Tibetan Buddhist nun who spent 15 years in prison on political charges was allowed to leave China and flew to the United States on Wednesday.

Phuntsog Nyidron was released from prison in 2004 but her movements were restricted and she was refused a passport until recently, said John Kamm, president of the Dui Hua Foundation in San Francisco. The foundation researches Chinese prisons and has helped arrange prisoner releases.

Her departure from China comes ahead of a visit to Washington by Chinese President Hu Jintao. But Kamm, who has helped to arrange the release of other prisoners, said he didn't know whether the decision to let her leave was connected to Hu's trip.

"The U.S. will view this quite positively, of course," Kamm said by phone from San Francisco. He noted the trip comes in the midst of a U.S.-Chinese human rights dialogue and changes to the U.N. system of human rights monitoring, and said both might have influenced the decision.

Phuntsog Nyidron arrived in San Francisco accompanied by a U.S. diplomat. Kamm said she would eventually fly on to Washington, where she was expected to be met by another Tibetan nun and fellow former prison inmate.

After arriving in Washington, Phuntsog Nyidron said Chinese officials told her not to discuss her situation, the U.S.-based broadcaster Radio Free Asia reported.

"Although I was released from prison in 2004, I suffered many hardships, as did my family members. I developed three different ailments," she was quoted as saying. "I was also told by the Chinese authorities not to discuss my situation since my family remain in Tibet."

Phuntsog Nyidron was a nun at the Michungri nunnery when she was arrested at age 22 in 1989 on charges of "counterrevolutionary propaganda and incitement" and sentenced to eight years in prison.

In 1993, she and 13 other women became known as the "singing nuns" after they used a tape recorder smuggled into Drapchi Prison in the Tibetan capital of Lhasa to record songs about their love for their families and their homeland. Their sentences were extended after the tape was smuggled out of the prison.

Phuntsog Nyidron was cited in speeches on human rights by the U.S. ambassador to Beijing, Clark T. Randt, and American lawmakers wrote to the Chinese government appealing for her release.

She was the last member of the group to be released.

After she left prison, she was barred from re-entering a nunnery, speaking with the press or meeting freely with foreigners, according to Kamm. He said police in Lhasa placed her under surveillance.

Kamm expressed optimism that the decision to grant Phuntsog Nyidron a passport would lead other former prisoners being allowed to obtain travel documents and to leave the country.