A Chinese group has hacked into America's largest business lobbying organization, compromising all data on its servers and information about its three million members, the Wall Street Journal reported Wednesday.

The illicit operation against the US Chamber of Commerce, discovered and shut down in May 2010, the report said, was one of the most serious security breaches in a years-long confrontation between US firms and Chinese hackers intent on cracking into companies and accessing sensitive or confidential data.

People familiar with an internal chamber investigation into the breach said the operation, involving up to 300 different Internet addresses, had been conducted by a group which US officials suspect of having ties to the Chinese government.

The Federal Bureau of Investigation reportedly told the chamber last year that the hackers were stealing its information, and while it is unclear just how damaging the digital break-in was, it was clearly well-organized and may have had access to the chamber's computer system for six months.

"What was unusual about it was that this was clearly somebody very sophisticated, who knew exactly who we are and who targeted specific people and used sophisticated tools to try to gather intelligence," the chamber's chief operating officer David Chavern told the Journal.

The US Chamber of Commerce, which has some 450 employees, more than 100 affiliates around the globe and represents the interests of American businesses in Washington, said it has taken extensive steps to revamp its security, and that investigators have uncovered no evidence of harm to its members or the group.

People familiar with the probe said the attack focused on four chamber employees who worked on Asia policy, and that several weeks of their emails had been stolen.

Some of the stolen messages included the names of companies and key figures in contact with the chamber, as well as trade policy documents, trip reports, and schedules.

"This is a different level of intrusion" than most hacking cases, the information officer Stan Harrell told the WSJ.