WASHINGTON (Reuters) - The Pentagon on Tuesday said computer hackers gained access to an unclassified e-mail system in the office of Defense Secretary Robert Gates, but declined comment on a report that the Chinese army was responsible.
The security breach occurred late last spring when Defense Department monitors detected the penetration of "elements of an unclassified e-mail system" that was immediately taken off line, Pentagon spokesman Bryan Whitman told reporters.
The e-mail system, located in the office of the secretary of defense, did not return to full operation for up to three weeks.
"There was never any threat to the classified systems," Whitman said.
"There was no disruption to (defense) operations or adverse impact to ongoing operations that the department was conducting ... all precautionary measures were taken and the system was restored to service," he said.
Whitman spoke after the Financial Times newspaper quoted current and former U.S. officials as saying that Chinese People's Liberation Army hackers broke into a Defense Department network in June and removed data.
China rejected the report's claims.
"The Chinese government has consistently opposed and vigorously attacked according to the law all Internet-wrecking crimes, including hacking," Foreign Ministry spokeswoman Jiang Yu said in Beijing.
"Some people are making wild accusations against China ... They are totally groundless and also reflect a Cold War mentality," she said.
The Financial Times cited one source familiar with the Pentagon incident as saying there was a "very high level of confidence ... trending towards total certainty" that the Chinese army was behind it.
Beijing has devoted a large part of its rising defense budget to developing more advanced technology, including computer capabilities.
But Whitman declined to comment on the hackers' suspected origins and other details of the incident.
"It is often very difficult to pinpoint the true origin of a particular intrusion," Whitman said.
"Even if you have some degree of confidence in origin, attaching origin to -- for example -- a nation state or an authorized activity of a government, that's a wholly different kind of thing," he said.
Hackers attempt to probe the Pentagon's Global Information Grid hundreds of times a day, he said, adding that major incidents are turned over to U.S. law enforcement and counterintelligence for investigation.
The Financial Times report came a week after German Chancellor Angela Merkel raised similar claims that Chinese hackers had infected German government ministries with spying programs.
Germany's Der Spiegel magazine reported last month that hackers believed to be linked to the Chinese army had gained access to Merkel's office and the German Foreign Ministry. China's Foreign Ministry denied that report as well.
Additional reporting by Chris Buckley in Beijing and Kate Kelland in London