Google has one-third of the Chinese search market
The US state department says it will make a formal protest to China over alleged cyber attacks on the internet search giant, Google.
A spokesman said the US would demand an explanation in the coming days.
Google this week threatened to pull out of China because of what it said were hacking attacks and censorship; Beijing has tried to play down the threat.
Another US internet giant, Yahoo, is also reported to have been targeted by hackers in China.
"We will be issuing a formal demarche to the Chinese government in Beijing on this issue in the coming days," said state department spokesperson PJ Crowley.
"It will express our concern for this incident and request information from China as to an explanation of how it happened and what they plan to do about it."
There are also reports that Yahoo, another US search engine, had noticed it had been a target of Chinese hacking attacks, prior to Google's public acknowledgment of its own fears.
However, Yahoo has not given any official confirmation of this.
In response to Google's concerns, China has said that foreign internet firms are welcome to do business there "according to the law".
Google had stated that cyber attacks originating in China aimed at rights activists, and increased web censorship, might force it to end its China operations.
Jiang Yu, a Chinese foreign ministry spokeswoman, insisted the internet was "open" in China.
Google announced late on Tuesday that it was no longer willing to censor its Chinese search engine - google.cn.
The search engine said it would hold talks with the government in the coming weeks to look at operating an unfiltered search engine within the law in the country, though no changes to filtering have yet been made.
When Google launched google.cn in 2006, it agreed to censor some search results - such as the 1989 Tiananmen Square protests, Tibetan independence or Falun Gong - as required by the Chinese government.
Google currently holds about one-third of the Chinese search market, far behind Chinese rival Baidu, which has more than 60%.
HAVE YOUR SAY
If Google pulls out then the Chinese people will be left with only local search engines
China has more internet users - about 350 million - than any other country and provides a lucrative search engine market worth an estimated $1bn (£614m) last year.
The CEO of Microsoft has said that the software giant had no plans to pull out of China, and played down Google's fears.
"I don't understand how that helps anything. I don't understand how that helps us and I don't understand how that helps China," said Steve Ballmer.
"There are attacks every day. I don't think there was anything unusual," Mr Ballmer added.