BEIJING, Nov 18 (AFP) - An imprisoned Tibetan nun, whose original conviction was declared unlawful by a US committee, has had nine years added to her jail term for insubordination, a human rights watchdog said Monday.

The additional sentence was passed on Ngawang Sangdrol, 19, for refusing to stand up in front of a prison guard during a re-education drive in Lhasa's Drapchi prison, and for shouting "Free Tibet" during a punishment period, the Tibet Information Network (TIN) said, citing unofficial sources.

The extra term was announced on July 31, the sources said.

Sangdrol, who comes from Garu nunnery five kilometers (three miles) north of Lhasa, will have the new sentence added to the two she is already serving -- three years for taking part in a 1992 pro-independence demonstration and six years for singing nationalist songs in prison in 1993.

The accumulated sentence of 18 years means Sangdrol is now facing longer in jail than any other female political prisoner in Tibet, similar in length to the sentences served by Tibetans in the "hardline" era of the 1960s and 1970s.

In November last year, the UN Working Group on Arbitrary Detentions, which assesses reports of illegal imprisonment, ruled that the continuing detention of Sangdrol was arbitrary because she had been punished for exercising her right to freedom of opinion.

The committee asked China to "take the necessary steps to remedy the situation in order to bring it into conformity with the provisions and principles incorporated in the Universal Declaration of Human Rights."