Chinese paramilitary police's field kitchen in Hutiaoxia, or Tiger Leaping Gorge, southeast of Zhongdian, 20 Mar 2008

Witnesses and foreign media have reported a huge buildup of Chinese troops in areas with large Tibetan populations, as the government cracks down on an unprecedented wave of protests.

Foreign media report seeing troop convoys with hundreds of vehicles moving by road and rail into Tibet on Wednesday and Thursday. Witnesses report a heavy security force buildup and roadblocks in Tibetan areas in the adjoining provinces, including Yunnan, where no protests have been reported.

In Lhasa, Chinese authorities forced the last known foreign journalist in the Tibetan capital to leave Thursday. German correspondent Georg Blume reported seeing thousands of security personnel on the streets of the city carrying out house-to-house searches.

Beijing acknowledged Thursday for the first time that protests have spread from the Tibet Autonomous Region to nearby provinces where more than half of China's Tibetan population lives.

China's official Xinhua news agency says police shot and wounded four rioters in a Tibetan area of Sichuan province earlier this week. It says police opened fire in self-defense.

One of the photos released by Tibetan Center for Human Rights and Democracy

But rights groups have released photographs showing what appear to be corpses with bullet wounds that the groups allege are some of the 15 Tibetans shot and killed by police during the incident.

There were unconfirmed reports that the sporadic pro-independence protests that have been staged in Qinghai, Gansu and Sichuan provinces continued Thursday.

New videos of the recent protests emerged Thursday, including footage shot by Chinese security forces in Lhasa of the monk's demonstration that appears to have sparked the wave of protests.

Tibet's government-in-exile says at least 99 people have been killed in a Chinese crackdown on protesters. Beijing says Tibetan rioters have killed 13 civilians in Lhasa.

A state-run television station in Tibet showed footage of 24 men arrested in connection with the Lhasa protests. The prime minister of Tibet's government-in-exile, Samdhong Rinpoche, puts the number of those arrested since Sunday at more than 800.

Some information for this report was provided by AFP, AP and Reuters.