China stepped up its pressure on Tibetan protesters on Friday, releasing photos of wanted suspects who were captured on film in the worst rioting against Chinese rule in Tibet in nearly 20 years.

But with security forces having been poured into Tibet and other Tibetan-populated areas of China, rights groups and activists warned of mass arrests and the possible torture of those taken into custody.

With China keen to put its best face forward ahead of the Olympic Games in Beijing in August, US House of Representatives speaker Nancy Pelosi had harsh words for China as she met the Dalai Lama in India.

"The situation in Tibet is a challenge to the conscience of the world," said Pelosi, who was greeted by thousands of flag-waving Tibetan exiles as she arrived for talks Friday with Tibet' s exiled spiritual leader.

"What is happening, the world needs to know," she said.

Faced with international concern over its handling of the unrest that erupted last week against Chinese rule in Tibet, state media acknowledged for the first time that police had opened fire on protesters.

After days of official statements that no lethal force had been used to quash the unrest, which has left an unknown number dead, state news agency Xinhua reported late Thursday that four people had been shot and wounded.

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