The apparent blockage follows a series of warnings from Chinese authorities to Google management accusing Google of providing links to pornographic sites.

"We have found that Google has spread a lot of pornographic content, which is a serious violation of Chinese laws and regulations"

Qin Gang,
Chinese foreign ministry spokesman

The timing of the outage has raised speculation among some Chinese bloggers and internet monitors that the move was a warning shot from the government for Google to take action.

On Thursday a foreign ministry spokesman kept up the pressure on the search giant, repeating the government's position that Google was illegally spreading pornograhic content.

"We have found that Google has spread a lot of pornographic content, which is a serious violation of Chinese laws and regulations," Qin Gang told reporters.

"Google is an internet enterprise providing services in China and should earnestly abide by Chinese laws and regulations," he said, adding he had no specific details on the recent outage.

Monitoring system

China's communist government has the world's most extensive web monitoring and filtering system, and it regularly blocks access to foreign websites.

China has the world's most extensive web monitoring system [GALLO/GETTY] 
While the government says the main targets are pornography, online gambling, and other sites deemed harmful to society, critics say it often uses those claims as cover for detecting and blocking sensitive political content.

Earlier this month China ordered all computer manufacturers to ensure that every computer sold after July 1 is shipping with Chinese-developed internet filtering software, known as "Green Dam Youth Escort".

The order has proved controversial, with internet watchdogs saying the programme could be used to keep track of online dissent and discussion of sensitive political issues.

On Thursday the US government added its calls for Chinese authorities to revoke the order.

In a letter to Chinese officials, Gary Locke, the US secretary of commerce and Ron Kirk, the US trade representative said the move could violate China's free trade commitments and raise security risks for users.

'Untenable'

"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 was quoted as saying.

Chinese officials insist the Green Dam filtering software is aimed at protecting children by blocking access to violent or pornographic material online and can be uninstalled by users if they wish.

The US call for China to back down adds to an array of disputes between the major trading partners, coming a day after the US announced it was taking China to the WTO in a row over Chinese restrictions on the export of several commodities.

Last week, American officials met Chinese regulators and expressed concern about the effort to censor internet use and its possible trade impact.

Thursday's letter from the US commerce and trade secretaries also raised the possibility that Washington might challenge China's trade strategies.