SAN FRANCISCO/BEIJING (Reuters) - Google's threat to quit China over censorship and hacking intensified Sino-U.S. frictions on Wednesday as Washington said it had serious concerns and demanded an explanation from Beijing.

China has not made any significant comment since Google, the world's top search engine, said it will not abide by censorship and may shut its Chinese-language google.cn website because of attacks from China on human rights activists using its Gmail service and on dozens of companies, including Adobe Systems.