BEIJING â€"  Google has begun notifying Chinese users when they are using search terms that can trigger China's Internet blocks, in its boldest challenge in two years to Beijing's efforts to restrict online content.

The search giant unveiled on its Chinese site this week a new mechanism that identifies political and other sensitive terms that are censored by Chinese authorities.

For example, when users search for keywords like "carrot" -- which contains the character for Chinese president Hu Jintao's surname -- a yellow dropdown message says: "We've observed that searching for 'hu' in mainland China may temporarily break your connection to Google. This interruption is outside Google's control."

'We've observed that searching for 'hu' in mainland China may temporarily break your connection to Google. This interruption is outside Google's control.'

- Google warning to Chinese users

Google acknowledged on its official blog Thursday that users in China are having trouble accessing its services, saying failed searches can impair performance on the site. "Users are regularly getting error messages like 'This webpage is not available' or 'The connection was reset,' " the post said.

Google says it hopes the alerts "will help improve the search experience in mainland China," where Google's search and other services have been unstable since it entered a public spat with Chinese authorities over censorship over two years ago. A Google spokesman declined to comment further.

Chinese officials do not discuss their Internet restrictions, and its search terms are treated as state secrets. In its post, Google said the trigger terms were identified based on reviews of the outcomes of the 350,000 most popular search queries in China, not an official list.

The post does not mention censorship, or explicitly say Chinese authorities are the cause of the blocks.

Chinese foreign ministry spokesman Liu Weimin said at a regular press briefing Friday that "there are more than 500 million Internet users in China, and they have access to plenty of information ... Like other countries, China also administers its Internet according to law."

China's restrictions include the names of top leaders as well as high-profile dissidents like blind legal activist Chen Guangcheng and references to the 1989 Tiananmen Square protests and crackdown.

The move is the most significant Google has made related to China's Internet restrictions since early 2010, when it publicly said it would not adhere to China's censorship policies and said it might have to shut down its Google.cn China site as well as its offices there.

Google ultimately kept its China offices but moved its web search and other services to Hong Kong, where it does not have to comply with regulations in mainland China.

Read more on Google's struggles with China in the Wall Street Journal.