China's comprehensive Internet filtering regime for political, social or religious content is complex and opaque -- creating precarious conditions for providers, the U.S. Trade Representative's official said in its annual report to the U.S. Congress on Wednesday.

In a report compiled before Google Corp. moved its Chinese search portal to Hong Kong from China this month in a censorship dispute, the USTR said: changes to Internet filtering can occur without warning or public explanation.

Chinese government authorities may issue lists of banned search terms or banned sites weekly, with little justification or means of appeal, putting Internet-enabled services in a precarious position, caught between complying with the law and implementing apparently arbitrary restrictions, it said.

(Editing by Sandra Maler)