A senior US lawmaker, Nancy Pelosi, has called for an independent investigation into China's claims that the Dalai Lama instigated the violence in Tibet.
Ms Pelosi, Speaker of the US House of Representatives, also called on the international community to denounce Chinese rule in Tibet.
She spoke out while holding talks in northern India with the Dalai Lama.
The Chinese authorities are continuing to tighten security following days of protests by Tibetans.
China says 19 people have been killed by rioters in Lhasa, the main city.
The Tibetan government-in-exile - headed by the Dalai Lama, regarded by many Tibetans as their spiritual leader - says at least 99 people have died in the crackdown by Chinese troops.
Chinese officials have accused the Dalai Lama and his supporters of organising violent clashes in Tibet in an attempt to sabotage this summer's Beijing Olympics and promote Tibetan independence.
Correspondents say the protests have presented the biggest challenge to Chinese rule in Tibet in almost two decades.
Speaking in Dharamsala, seat of Tibet's government-in-exile, Ms Pelosi said: "We call upon the international community to have an independent outside investigation on accusations made by the Chinese government that His Holiness [the Dalai Lama] was the instigator of violence in Tibet."
She added: "The situation in Tibet is a challenge to the conscience of the world.
Chinese police and troops have converged on restive areas
"If freedom-loving people throughout the world do not speak out against China and the Chinese in Tibet, we have lost all moral authority to speak out on human rights."
Ms Pelosi said she was not seeking a boycott of the Beijing Olympics, but warned that the "world is watching" events in China.
Ms Pelosi is one of the sharpest critics of Beijing's human rights record in the US Congress.
Her visit at the head of a congressional delegation was planned before the protests began.
Rifles and bayonets
Anti-China protests began on 10 March in Lhasa and gradually escalated, spreading to Tibetan communities in neighbouring Gansu, Sichuan and Qinghai provinces.
Tibet: Protests began in Lhasa on 10 March, dozens reported dead over weekend
Gansu: Unrest spread to Machu, where Tibetan government in exile says 19 died, and near Hezuo, where protesters were filmed tearing down Chinese flag
Sichuan: State media says four people 'shot and wounded' by police in Aba
Qinghai: Other unrest reported
China is not allowing foreign journalists into Tibet. Troops have also sealed off towns in the surrounding areas where unrest has taken place, witnesses say.
But the BBC's James Reynolds spent 24 hours in Hezuo in Gansu, where earlier this week Tibetan protesters tore down the Chinese flag.
Chinese security forces had swamped the town and the streets were full of police cars, check points and military trucks.
On the southern entrance to Hezuo there were rows of soldiers carrying AK47 rifles and bayonets, our correspondent said.
Public notices and police broadcasts told protesters to surrender by midnight on 25 March or face arrest and punishment.
Other witnesses have reported seeing hundreds of troop carriers heading for Tibetan areas in recent days.
On Thursday Chinese authorities admitted for the first time that members of the security forces had fired on Tibetan protesters.
Police wounded four protesters "in self-defence" last Sunday in Aba county, a Tibetan area of Sichuan province, Xinhua news agency said.
An earlier Xinhua report said police had shot the four dead, but it was quickly changed.
Xinhua did not provide further details of the incident, but Tibetan activists say at least eight people were killed at a demonstration against Chinese rule near the Kirti monastery in Aba on Sunday.
Tibetan activists say police killed protesters in Aba
Earlier this week, the Tibetan Centre for Human Rights and Democracy circulated photos of bodies with apparent gunshot wounds, which it said were the result of police firing indiscriminately at protesters.
On Thursday, US Secretary of State Condoleezza Rice held telephone talks with her Chinese counterpart, Yang Jiechi, in which she urged Beijing to show restraint.
But Mr Yang told her the protesters were trying to sabotage both the Olympics and social stability - and reiterated China's position that it blamed the Dalai Lama for the violence.
The Dalai Lama - who in 1989 won the Nobel Peace Price for his commitment to non-violence in the quest for Tibetan self-rule - has called for talks with Chinese President Hu Jintao.