More than 1,000 anti-government protesters are missing in Tibet according to a human rights group.

Amnesty International said of the 4,000 people detained by police during riots three months ago, about a quarter are still unaccounted for.

The others have been either released or placed under formal arrest.

Thousands of monks, nuns, and other protesters took to the streets in March to demand independence or greater autonomy from China. Violent clashes erupted when the rallies and marches were put down by Chinese security forces.

Amnesty International researcher Roseanne Rife said that according to official figures there is a big gap between the number of people arrested - and those charged or released.

"There's a huge discrepancy. Hundreds - over a thousand - are unaccounted for," she said.

Asked what might have happened to them, Ms Rife pointed out that under Chinese law it is possible for someone to be held under administrative detention for four years without charge.

Chinese officials could not immediately be reached for comment on Amnesty's allegations.

Amnesty International has urged the Chinese government to provide information on those detained following the protests, and requested that independent observers be allowed free access to Tibet.

The calls come ahead of the arrival of the Olympic torch in the Tibetan capital of Lhasa this weekend.

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