Chinese troops and police have tightened their hold on Tibetan areas in the westernmost region of the country as they work to keep anti-government protests from spreading.

Journalists and activist groups have reported large numbers of troops in provinces along Tibet's eastern border.

Reporters Without Borders expressed outrage at the methods the Chinese authorities use to obstruct foreign journalists from covering the situation in Tibet.

The media rights group said that, for the repression in Tibet to end, the United Nations must demand the return of foreign journalists barred from the Tibetan areas and the dispatch of independent observers.

Peaceful protests against Chinese rule in Tibet began last week and gradually turned violent.

China says at least 16 people were killed in riots in the Tibetan capital of Lhasa Friday. But the Tibetan government-in-exile says at least 99 people have been killed in the unrest.

The government-in-exile says 19 Tibetans were shot dead by security forces during protests in Gansu province on Tuesday.

Tibetan groups have reported further pro-independence protests in Qinghai, Gansu and Sichuan and the Tibet Autonomous Region.

Human rights groups say hundreds of people have been arrested following last week's riots. The U.S.-based International Campaign for Tibet estimates at least 900 people were arrested in Lhasa on Saturday and Sunday.

China's official Xinhua news agency says 105 people involved in last Friday's riots have surrendered to police.