US objects to China's net filter
Ron Kirk has now objected to several aspects of Chinese trade policy
The US has called on China to scrap its plan to put net-filtering software on all its computers.
It said that China's proposals would violate its free trade obligations, weaken computer security and raise serious censorship concerns.
China has demanded that all computers come supplied with software called Green Dam from 1 July.
China has the world's largest net-using population, and the proposals have been criticised even within China.
The US's objections also came a day after it filed an unfair trade complaint to the World Trade Organization (WTO) over raw material exports.
The latest comment raises the concern about a broader trade war between the US and China over everything from computer security to chicken poultry imports.
"Mandating technically flawed Green Dam software and denying manufacturers and consumers freedom to select filtering software is an unnecessary and unjustified means to achieve that objective, and poses a serious barrier to trade," said US Trade Representative Ron Kirk.
The US is now complaining that putting such pressure on manufacturers to pre-install or supply the software would violate China's WTO free trade obligations.
"China is putting companies in an untenable position by requiring them, with virtually no public notice, to pre-install software that appears to have broad-based censorship implications and network security issues," said US Commerce Secretary Gary Locke.
The Green Dam Youth Escort software was created to stop people looking at "offensive" content such as *****graphic or violent websites, China has said.
But China's Ministry of Industry and Information Technology later said that use of the software was not compulsory and that it was possible to uninstall the program.
Tests carried out on Green Dam outside China also showed that it left PCs open to many different security risks, including being hijacked.
Petitions calling for Green Dam to be scrapped have circulated widely within China.
Meanwhile, it appeared that China had blocked users of Google in China from using the search engine.
Users in Beijing and Shanghai said they could not access Google's main site, its Chinese-language version nor its mail services.
"We have found that Google has spread a lot of *****graphic content, which is a serious violation of Chinese laws and regulations," Chinese Foreign Ministry spokesman Qin Gang said.
He urged the company to abide by local rules, but said he had no specific details on the outage.
The disruption comes a week after China accused Google of deliberately linking to "*****graphic and vulgar" content and ordered it to stop.
Google said it was looking to why there was an outage.