A U.S. security firm has linked China’s military to cyberattacks on more than 140 U.S. and other foreign corporations and entities, according to a report released Tuesday.

The 60-page study by investigators at the Alexandria-based Mandiant security firm presents one of the most comprehensive and detailed analyses to date tracing corporate cyber-espionage to the doorstep of Chinese military facilities. And it calls into question China’s repeated denials that its military is engaged in such activities.

The document, first reported by the New York Times, draws on information that Mandiant collected from what the company said was the systematic theft of data from at least 141 organizations over seven years. Mandiant traced the attacks back to a single group it designated “Advanced Persistent Threat 1,” or “APT1,” and now has identified the group as a Chinese military unit within the 2nd Bureau of the People’s Liberation Army General Staff Department’s 3rd Department, code named “Unit 61398.”

Although most of the targets were U.S. companies, a Mandiant official said APT1 also hit about a dozen entities that he described as smaller U.S. local, state and federal government agencies and international governmental organizations overseas.

Senior U.S. officials, including President Obama, have raised the issue of Chinese cyber attacks on commercial targets over the past year. White House press secretary Jay Carney declined to address the Mandiant report, but he said, “We have repeatedly raised our concerns at highest levels about cybertheft with senior Chinese officials, including the military, and we will continue to do so.”

Analysts have long linked the unit to the Chinese military’s 3rd Department, and to extensive cyber-espionage. But what Mandiant has done is connect the dots and add new ones by locating the Internet protocol addresses used in commercial cyberattacks, placing them on a map and linking that information to open-source data about people associated with the unit.

“Since 2006, Mandiant has observed APT1 compromise 141 companies spanning 20 major industries,” the firm said in its report. Of those victims, 87 percent “are headquartered in countries where English is the native language,” it said.

Mandiant did not name the victims but said 115 of them are located in the United States, two in Canada and five in Britain. Of the 19 others, all but two operate in English.

One apparent victim was Telvent, a Madrid-based technology company with U.S. headquarters in Rockville that enables energy suppliers and others to remotely control their operations, security researchers and company officials said. Telvent helps manage 60 percent of the flow of hydrocarbons in North America and Latin America, according to its Web site. Technology made by the company, owned by Schneider Electric, also helps control and monitor power grids around the world.

Separately Tuesday, Apple Inc. said the same hackers who targeted Facebook last month have also gone after Apple and infiltrated a small number of the company’s computers. Neither company publicly identified the nationality of the hackers.

The top sectors targeted by the APT1 cyber-espionage campaign, Mandiant said, are information technology, aerospace, public administration, satellites and telecommunications, and scientific research and consulting.

“We have figured things out in an unclassified way that the government has known through classified means,” said Richard Bejtlich, Mandiant chief security officer, adding that the company shared the study with U.S. intelligence agencies before it was released.

The unit is just one of dozens working for the Chinese military in cyber-espionage all over the country, analysts say. There are other units within the General Staff Department’s 2nd Department, which conducts military intelligence, and within the Ministry of State Security, which conducts internal counterintelligence and external espionage, according to analysts.

APT1, also dubbed “Comment Crew” by security companies that have studied its tactics, focuses on commercial targets overseas, which makes its work more visible to the security firms tracking the intrusions. Chinese units that focus on military and intelligence targets are less visible to the cybersecurity companies.

The Chinese military has repeatedly denounced accusations that it is engaging in cyber-espionage, and did so again Tuesday.

“Similar to other countries, China faces serious threats from cyberattack and is one of the main victims of cyberattacks in the world,” the Ministry of Defense said. “The Chinese army never supported any hacking activities. The accusation that the Chinese military engaged in cyberattacks is neither professional nor in accordance with facts. “

Chinese Foreign Ministry spokesman Hong Lei on Tuesday also challenged the report’s findings. “Hacking attacks are transnational and anonymous,” and determining their origins is extremely difficult, he said. “We don’t know how the evidence in this so-called report can be tenable.”

Mandiant investigators said they based their conclusion in part by tracing an overwhelming number of cyberattacks by the APT1 group to networks serving a small area on the edges of Shanghai — the same area where Unit 61398 is believed to be operating in a 12-story building. It also found evidence that China Telecom had provided special high-speed fiber optic lines for those headquarters in the name of national defense.

The Mandiant report echoed a classified National Intelligence Estimate by the U.S. intelligence community that concluded that China was the most aggressive perpetrator of a massive campaign of cyber-espionage against commercial targets in the United States.

Nakashima reported from Washington. William Branigin in Washington contributed to this report.