China accused Google Inc. on Thursday of spreading pornography, a day after Chinese users reported that they were experiencing problems connecting to the site.

"We have found that the English version of google.com has spread lots of pornographic, lewd and vulgar content, which is in serious violation of Chinese laws and regulations," said foreign ministry spokesman Qin Gang at a news briefing.

He said authorities had summoned Google representatives and told them to "remove the material immediately."

Chinese internet users were unable to connect to Google's main search site or its China-based service, google.cn, beginning Wednesday evening.

Qin did not respond to questions about whether the government was responsible for the outage. But he said he hoped the problem can be "resolved immediately."

'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.'— U.S. Commerce Secretary Gary Locke

Meanwhile, U.S. Commerce Secretary Gary Locke and Trade Representative Ron Kirk, in a letter to Chinese officials, called on Beijing to revoke its order for the "Green Dam Youth Escort" filtering software to be pre-installed or supplied on a disc with all new PCs in China starting July 1. They warned the rule "poses a serious barrier to trade" and said the software might pose security risks.

"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," Locke said in a statement.

China encourages internet use for education and business, and has the largest population of internet users at more than 298 million. But the Communist government tries to block material deemed obscene or subversive and operates the world's most extensive web monitoring and filtering system.

WTO complaint

Locke and Kirk's letter said Beijing might have violated World Trade Organization rules that require governments to give companies advance notice of rule changes, an explanation and time to comment.

Locke and Kirk's letter raised the possibility that Washington might challenge China's rule in the WTO. The United States and European Union filed WTO complaints Tuesday accusing China of improperly favouring its domestic industries by restricting exports of industrial raw materials.

'Protecting children from inappropriate content is a legitimate objective, but this is an inappropriate means and is likely to have a broader scope.'— U.S. Trade Representative Ron Kirk

Chinese officials insist the filtering software is aimed at blocking access to violent or pornographic material online. Chinese web users have appealed to the government to repeal the order, pointing out that "Green Dam" mistakenly blocks access to online cartoons, pictures of animals and other innocuous subjects.

Researchers at the University of Michigan who studied "Green Dam" say they have found "serious security vulnerabilities due to programming errors" that could allow any website a PC user visits to take control of the computer.

"Protecting children from inappropriate content is a legitimate objective, but this is an inappropriate means and is likely to have a broader scope," Kirk said.

"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," he said.

Washington and Beijing have had a series of technology-related disputes over China's effort to restrict internet access and use regulation to promote development of Chinese high-tech industry.

Last year, Beijing ordered foreign sellers of computer security technology to disclose how their products work. Following U.S. protests, the government agreed in April to postpone that for a year. The order still applies to products sold to Chinese government agencies.