Google on Thursday tuned its search engine in mainland China to tip people off when they try to use characters that evidently prompt censors to derail queries.

The change resulted from complaints that Google search service in China was inconsistent or unreliable, according to Google senior vice president of knowledge Alan Eustace.

"We've taken a long, hard look at our systems and have not found any problems," Eustace said in a blog post.

"However, after digging into user reports, we've noticed that these interruptions are closely correlated with searches for a particular subset of queries."

Popup messages tell users in China when they enter query keywords that may cause "connection issues," according to Eustace.

The California-based Internet giant does not suggest alternative search terms. It is left to users, if they are so inclined, to come up with alternative characters or words to continue Internet searches.

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

"Of course, if users want to press ahead with their original queries they can carry on."

Simplified Chinese language characters more often triggered connection trouble, according to Google.

Searches using characters that also happened to be in names of current or former national leaders, for example one meaning "river," also were prone to delays in getting results.

"We've observed that many of the terms triggering error messages are simple everyday Chinese characters, which can have different meanings in different contexts," Eustace said.

When alerted to a problematic query term, users are given the option to click on an "interruption" link that leads to a help center.

Two years ago, Google effectively shut down its Chinese search engine, redirecting mainland users to a landing page that provides a door to its uncensored site in Hong Kong.

Google at the time said the move resulted from censorship and cyberattacks.

The change came after months of tension between Google and Beijing over the US firm's efforts to skirt an army of government censors who police the Web for content deemed inappropriate or unacceptable.

Beijing has denied any role in the cyberattacks which Google said had targeted Gmail accounts of Chinese dissidents and deemed it "totally wrong" to stop filtering its Chinese-language search engine.