At a news conference in Beijing on Wednesday, the ministry suggested that the allegations were destructive and challenged the study, which was produced by Mandiant, an American computer security company. The report identified P.L.A. Unit 61398 in Shanghai as one of the most aggressive computer hacking operations in the world. Geng Yansheng, a spokesman for the Ministry of National Defense, said that China had been the victim of cyberattacks that have originated in the United States and that Mandiant had mischaracterized China's activities. "Chinese military forces have never supported any hacking activities," Mr. Geng said at the briefing. "The claim by the Mandiant company that the Chinese military engages in Internet espionage has no foundation in fact." On Tuesday, a spokesman for China's Ministry of Foreign Affairs, Hong Lei, made similar remarks, arguing that cyberattacks were difficult to trace because they were "often carried out internationally and are typically done so anonymously." The New York Times reported on Tuesday that a growing body of digital forensic evidence pointed to the involvement of the army unit in Shanghai and that American intelligence officials had also been tracking the unit's activities. On its Web site, Mandiant released a long report on Tuesday detailing some of its evidence, including Internet protocol addresses and even the identities of several Chinese individuals it thinks were behind some of the attacks. Mandiant said it had monitored the hackers as they logged onto social networking sites or through e-mail accounts.
Attempts to contact two of the individuals through phone numbers and instant message services were unsuccessful. In one case, an individual, whose online profile said he was 28 years old and a graduate of a university that specializes in computer science, declined to answer questions.
Several military analysts said they had also traced some serious cyberattacks back to the People's Liberation Army and its Shanghai Unit 61398, which is known to be engaged in network security.
Still, many security experts concede that it is difficult if not impossible to know for certain where attacks originate, because hackers often take control of computers in various places.
Chinese officials have insisted in recent years that China is one of the biggest targets of cyberattacks.