China uses the so-called "Great Firewall of China" to control access
Access to Google has been disrupted in some parts of China, amid a row over what Chinese citizens should be allowed to view over the internet.
Users reported they could not access either Google's search engine or its Chinese-language version.
Chinese Foreign Ministry spokesman Qin Gang accused Google of spreading pornography and breaking Chinese law.
The move came as the US called on China to scrap its plan to put net-filtering software on all new computers.
China has demanded that all computers come supplied with software called Green Dam Youth Escort from 1 July, which it says would filter out pornographic content.
Separately, google.com and some of its products, such as its mail service, were not available in China from Wednesday night to Thursday morning Beijing time, according to Chinese portal Sohu.com.
Most users were able to connect on Thursday, though it was unclear exactly how widespread the disruption actually was. Google said it was investigating the outage.
The disruption to Google's services reported by users in Beijing and Shanghai comes a week after China accused Google of deliberately linking to "pornographic and vulgar" websites and ordered it to stop.
"We have found that Google has spread a lot of pornographic content, which is a serious violation of Chinese laws and regulations," Mr Qin told reporters on Thursday.
He urged the company to abide by local rules, but said he had no specific details on the outage.
Ron Kirk has now objected to several aspects of Chinese trade policy
Meanwhile, the US said China's proposed internet filter would violate China's free trade obligations, weaken computer security and raise serious censorship concerns.
"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 latest comment raises the concern about a broader trade war between the US and China over everything from computer security to chicken poultry imports.
It came a day after it filed an unfair trade complaint to the World Trade Organization (WTO) over raw material exports.
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.
Quentin Sommerville, BBC Beijing correspondent
The software, Green Dam Youth Escort, has been heavily criticised by Chinese internet users, and even parts of the state media.
It appears to have been badly written, and parts of it may have been lifted from a rival US software filter.
Critics says it will put computers at greater risk from hackers.
It works by looking for fleshtones, as well as keywords, but has also been found to block sites which contain lots of the colour pink.
"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 pornographic 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, which has the world's largest net-using population.