The American government has cited Chinese Internet controls as a trade barrier in a report that comes as Beijing tries to block its public from seeing news about the finances of Chinese leaders' families.
Chinese filters, which block access to websites including the Google search engine and social media such as Twitter, are a "significant burden" on businesses, the US Trade Representative said in an annual report on trade conditions.
It gave no indication Washington plans to take action but highlights the economic cost of pervasive Chinese censorship that also draws criticism from human rights and pro-democracy activists.
On such issues, Washington is at odds with Beijing, which sees strict control over information as essential to protecting the Communist Party's monopoly on power.
China restricts access to online materials by requiring traffic to pass through state-controlled gateways linked to the global Internet. Controls have been tightened since President Xi Jinping became party leader in 2012.
The filters, known informally as the Great Firewall of China, are in line with Beijing's advocacy of "Internet sovereignty," or allowing governments to impose control on the freewheeling Internet within their borders.
Xi called in a speech in December for the creation of a global "governance system" for cyberspace.
This week, Chinese Web users have been blocked from seeing news reports about documents from a law firm in Panama that say relatives of political figures including Xi own offshore companies.
State media have carried brief reports on the revelations but with no mention of Chinese figures.
Chinese regulators block access not just to websites operated by human rights or pro-democracy activists but also to dozens of news, entertainment and social media services that operate freely in other countries.
"Outright blocking of websites appears to have worsened over the past year, with eight of the top 25 most-trafficked global sites now blocked in China," the USTR says in its National Trade Estimate.
It said much of the blocking appears to be arbitrary, including a home improvement website in the United States. Foreign and local companies in China that rely on the Internet for sales, accounting and other internal functions complain the filters hinder their operations.
Almost 80 per cent of companies that responded to a survey released in January by the American Chamber of Commerce in China said they were "negatively impacted" by the controls.
