Journalists are popular targets, particularly in efforts to determine what information reporters have and who may be talking to them.
The Chinese foreign and defense ministries called the Times' allegations baseless, and the Defense Ministry denied any involvement by the military.
"Chinese law forbids hacking and any other actions that damage Internet security," the Defense Ministry said. "The Chinese military has never supported any hacking activities. Cyberattacks are characterized by being cross-national and anonymous. To accuse the Chinese military of launching cyberattacks without firm evidence is not professional and also groundless."
In a report in November 2011, U.S. intelligence officials for the first time publicly accused China and Russia of systematically stealing American high-tech data for economic gain. And over the past several years, cybersecurity has been one of the key issues raised with allies as part of a broader U.S. effort to strengthen America's defenses and encourage an international policy on accepted practices in cyberspace.
U.S. cybersecurity worries are not about China alone. Administration officials and cybersecurity experts also routinely point to widespread cyberthreats from Iran and Russia, as well as hacker networks across Eastern Europe and South America
The U.S. itself has been named in one of the most prominent cyberattacks — Stuxnet — the computer worm that infiltrated an Iranian nuclear facility, shutting down thousands of centrifuges there in 2010. Reports suggest that Stuxnet was a secret U.S.-Israeli program aimed at destabilizing Iran's atomic energy program, which many Western countries believe is a cover for the development of nuclear weapons.
The White House declined comment on whether it will pursue aggressive action on China.
"The United States has substantial and growing concerns about the threats to U.S. economic and national security posed by cyber intrusions, including the theft of commercial information," said spokesman Caitlin Hayden. "We have repeatedly raised our concerns with senior Chinese officials, including in the military, and we will continue to do so."
Cybersecurity experts have been urging tougher action, suggesting that talking with China has had no effect.
"We need to find new approaches if we want to dissuade this type of activity," said Stewart Baker, former assistant secretary at the Homeland Security Department and now in private law practice with Steptoe and Johnson in Washington. He said the U.S. must do a better job of attributing the cyberattacks to particular groups or nations and "see if we can sanction the people who are actually benefiting from them."
The Obama administration has slowly been ratcheting up its rhetoric. In an unusually strong speech last October, Panetta warned that the U.S. would strike back against cyberattacks, even raising the specter of military action. And the White House has been urging Congress to authorize greater government action to protect infrastructure such as the nation's electric grid and power plants.
Alan Paller, director of research at SANS Institute, a computer-security organization, said that the level of cyberattacks, including against power companies and critical infrastructure, has shot up in the last seven or eight months. And the U.S. is getting more serious about blocking the attacks, including an initiative by the Defense Department to hire thousands of high-tech experts.
Just talking about it, he said, is having no effect.
Lewis, who has met and worked with Chinese officials on the issue, said their response has been consistent denial that China is involved in the hacking and counter-accusations that the U.S. is guilty of the same things.
"In the next year there will be an effort to figure out a way to engage the Chinese more energetically," he said. "The issue now is how do we get the Chinese to take this more seriously as a potentially major disruption to the relationship."
The answer, he said, is, "You have to back up words with actions, and that's the phase I think we're approaching."
Associated Press writers Bradley Klapper in Washington and Michael Lietdke in San Francisco contributed to this report.