Secretary of State Hillary Rodham Clinton says the U.S. will "candidly and consistently" address its differences with China over internet freedom.

Clinton urged China to conduct a thorough investigation of the cyber attacks against Google and seemed to support Google's planned pullout from China, in a speech Thursday morning in Washington, D.C.

"American companies need to make a principled stand," said Clinton.

Google says the Chinese government attempted to steal its code and to hack the Gmail accounts of human rights activists.

Clinton said America stands for a single internet where all of humanity has access to knowledge and ideas.

She criticized China, Tunisia and Uzbekistan for recently "stepped up censorship of the internet." She also condemned Vietnam for blocking a popular social media site and Egypt for arresting bloggers.

"A new information curtain is descending over much of the world," said Clinton.

Clinton compared the freedom to connect online to the freedom of assembly guaranteed in the UN Universal Declaration on Human Rights and the U.S. Constitution.

She said countries that restrict or filter internet access contravene the Universal Declaration on Human Rights.

"Countries that restrict free access to information or violate the basic rights of internet users risk walling themselves off from the progress of the next century," she said.

Clinton cited the use of texting in Haiti to find people trapped under rubble following the earthquake as an example of how information networks are being used to help, but warned that the same technology "can be harnessed for good or for ill."

She urged action on internet freedoms so that users around the world are not "buried in the rubble of oppression."

Clinton said the U.S. will support software developers making new tools to allow people to circumvent politically motivated censorship.

Earlier Thursday, China's Vice Foreign Minister He Yafei said Google's dispute with China and its threat to pull out of the country should not affect China's diplomatic ties with the United States.

"The Google case should not be linked with relations between the two governments and countries; otherwise, it's an over-interpretation," He told a news conference, according to the official Xinhua News Agency.

China has the world's largest internet population — 382 million people — but government filters, commonly known as the Great Firewall of China, block sites such as Facebook, YouTube and Twitter.

Google has said it will stop censoring its search results on Google.cn, set up in 2005, and has threatened to pull its business out of China altogether.

Google is the No. 2 search engine in China, behind Baidu.com, which has 60 per cent of the market there.

With files from The Associated Press