Google has fired a new salvo in a censorship battle with Beijing by adding a feature that warns users in China each time they enter keywords into its search engine that might produce blocked results and suggests they try other terms.

Google's announcement Thursday described the change as a technical improvement and made no mention of Beijing's extensive internet controls. But it comes after filters were tightened so severely in recent weeks that searches fail for some restaurants, universities or tourist information.

Authorities were trying to stamp out talk about an embarrassing scandal over the fall of a rising Communist Party star.

Google Inc. closed its China-based search engine in 2010 to avoid co-operating with government censorship. Mainland users can see its Chinese-language site in Hong Kong, but the connection breaks if they search for sensitive terms.

The new feature will alert users if they enter a search term that "may temporarily break your connection to Google," said a blog post by a Google senior vice-president, Alan Eustace.

He said it will suggest they "try other search terms."

"By prompting people to revise their queries, we hope to reduce these disruptions and improve our user experience from mainland China," Eustace wrote.

Former president's name flagged

Google cited as an example the Chinese character "jiang," or river, without mentioning it is the name of former president Jiang Zemin, the possible reason results are blocked. It says the site will recommend removing the character.

Google could anger Beijing by pointing out individual terms that might produce blocked results. Chinese regulators do not disclose which terms are banned. They try to hide censorship by returning the same error message as for a technical failure, possibly to avoid drawing attention to unwanted topics.

A Google spokesman declined to comment on whether the company was concerned about Chinese government retaliation.

Google was allowed to keep a network of advertising sales offices in China that might be vulnerable if the communist government tries to punish the company.

Google, based in Mountain View, Calif., had 16.6 per cent of China's search market in the first quarter based on use of its global and Hong Kong sites, according to Analysys International, a Beijing research firm. It was in second place behind local rival Baidu Inc., which has 78.5 per cent of the market, but ahead of other Chinese competitors.

Google is also promoting its Android mobile phone operating system for use by Chinese manufacturers. Beijing approved Google's $12.5 billion acquisition of Motorola Mobility, a wireless device maker, last month on condition Android remains available to Chinese companies and others at no cost for five years.

Tech companies uncomfortable with China's heavy hand

Tensions over censorship highlight Beijing's complicated relations with global technology companies. The communist government wants to boost incomes by promoting high-tech industry but insists on controlling access to information.

Beijing promotes internet use for education and business and has the world's biggest population of internet users, with 513 million people online as of December 2011, but tries to block politically sensitive material.

The latest tightening of controls was prompted by a flurry of rumours online about the downfall of Bo Xilai, a prominent politician who was party secretary of the major city of Chongqing in the southwest.

In addition to Bo's name, blocked terms include Chongqing and Yangtze River, which flows past the city. That means searches for universities, hotels, restaurants or other businesses that use those names also fail.

China's two most popular microblog services stopped allowing new postings for three days in early April to erase what they said were illegal or harmful postings.

Google's engineers reviewed the 350,000 most popular search queries in China in an effort to find "disruptive queries," the company said. Google gave no indication when development of the latest feature started but said it received reports of unreliable searches "over the past couple of years."

Google Sync service for Blackberry ending

Also on Thursday, Google announced it would be ending its Sync service for Blackberry smartphones starting June 1. The service allows users to synchronize their Google and Blackberry email, contacts and calendars.

Existing users can continue using the service, but it will no longer be available for download.

In a blog post, the company encouraged users to switch to BlackBerry Internet Service.