The White House says it is deeply concerned over the deadly violence in China's western Xinjiang region and has called for restraint.
Spokesman Robert Gibbs said from Moscow Monday that it is unclear what caused the riots and refused to comment further.
Chinese officials say at least 156 people have been killed and more than 800 hurt in riots between ethic Uighurs and police in Xinjiang.
Monday's fighting grew out of what witnesses say was a peaceful anti-government protest by Uighurs Sunday.
No one is sure what led the march to explode into violence. But Chinese officials accuse exiled Uighur groups of instigating the riots.
The groups deny involvement. They say the violence is a result of pent-up frustration with what the mainly Muslim Uighurs say is excessive control over their lives by the Han Chinese.
The Han are China's dominant ethnic group and include the leadership in Beijing and many regional authorities.
Chinese officials have not said how many of the dead in Xinjiang are Han and how many are Uighurs. But witnesses report seeing a large number of wounded Han.
United Nations Secretary General Ban Ki-moon is urging China to peacefully resolve its differences through talks.
China's state-run Xinhua news agency says police also broke up a crowd of demonstrators outside the main mosque in the Xinjiang city of Kashgar Monday.
Tensions between Han and Uighurs blew up last month after a false rumor spread throughout a toy factory in Guangdong that Uighur workers raped two Chinese girls. A huge fight broke out, killing two people.
Chinese authorities have accused the Uighurs of seeking independence for Xinjiang, where they are the majority ethnic group. Xinjiang borders a number of Central Asian states with large Muslim populations, including Kazakhstan, Kyrgyzstan, and Tajikistan. The Uighurs say the government exaggerates the threat of an independence movement in order to justify its control.
Some information for this report was provided by AFP, AP and Reuters.