The world's most popular search engine has been dragged into what appears to be an escalating row between China and the United States on Internet controls.

Beijing yesterday stepped up accusations that Google is spreading obscene content over the Internet, a day after US officials urged Beijing to abandon plans for the installation of a controversial porn-filtering software, Green Dam, on new computers.

The Foreign Ministry yesterday accused Google's English-language search engine of spreading obscene images that violated the nation's laws, less than 24 hours after disruptions to the company's search engine and other services within China.

Spokesman Qin Gang did not directly say whether official action was behind the disruptions, but made plain the government's anger and said "punishment measures" taken against Google were lawful.

"Google's English language search engine has spread large amounts of vulgar content that is lewd and pornographic, seriously violating China's laws and regulations," he told a regular news conference.

Qin said authorities "summoned representatives of Google in China and urged them to remove the content immediately".

The Internet Society of China last week ordered Google to block overseas websites with "pornographic and vulgar" content from being accessed through its Chinese-language version.

Late on Wednesday evening, Internet users in China were unable to access several Google services for up to two hours.

A spokesman for Google in China declined to comment.

Separately, US Commerce Secretary Gary Locke and US Trade Representative Ron Kirk on Wednesday voiced concerns over the "Green Dam" software in a letter to Chinese officials.

"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.

A source from the Ministry of Industry and Information Technology (MIIT) told China Daily yesterday that "issues related to Green Dam porn-filtering software are no longer only domestic, they have become diplomatic".

The source also said MIIT has been working closely with the Ministry of Commerce and the Ministry of Foreign Affairs to discuss issues regarding the software, adding that a statement will be released when all parties reach agreement.

The government has said that Green Dam, which limits access to violent and pornographic websites, has to be included with all computers sold on the Chinese mainland starting July 1.

Several legal and technical experts have said the government would revise, or even scrap, its directive as the software developers are facing copyright infringement claims from a US software developer and the software itself contains severe security vulnerabilities.

In yesterday's letter, the US officials invited China to discuss with industry and government officials ways to promote parental control "without restricting user choice, freedom of expression and the free flow of information".