BEIJING - US companies in China are worried about the security of commercial correspondence, a business group said Thursday, after Internet giant Google said China-based cyberspies had attacked their systems.

While not mentioning Google by name, the American Chamber of Commerce in China (AmCham) said preliminary survey results showed "members ... are concerned about the security of commercial correspondence, data and networks."

Yet the US companies surveyed by AmCham said they remained "optimistic about the prospects for China over both the medium and long term," AmCham said in a statement in response to questions posed by AFP.

The statement did not say when the business climate survey was conducted.

Google vowed Tuesday to stop bowing to Chinese Internet censors and risk banishment from the lucrative market, in protest against "highly sophisticated" cyberattacks aimed at Chinese human rights activists.

The China-based hackers struck Google and reportedly more than 30 other firms in an apparent bid for computer source codes, intellectual property, and information about human rights activists around the world.

AmCham said the "free flow of information, protection of intellectual property rights and uniform application of the law facilitates commercial activity for both Chinese and foreign companies."

The group encouraged "all parties, including the US and Chinese governments, to work toward these objectives."