The Wall Street Journal has become the second major US media organization to accuse Chinese hackers of targeting its computers in an apparent effort to spy on journalists covering China.
The announcement on Thursday came a day after The New York Times said hackers, possibly connected to China's military, had infiltrated its computers in response to its expose of the vast wealth amassed by a top leader's family.
The Journal reported that the attacks were "for the apparent purpose of monitoring the newspaper's China coverage" and suggested that Chinese spying on US media has become a "widespread phenomenon."
"Evidence shows that infiltration efforts target the monitoring of the Journal's coverage of China, and are not an attempt to gain commercial advantage or to misappropriate customer information," said a statement from Journal parent Dow Jones, a unit of Rupert Murdoch's News Corp.
Secretary of State Hillary Clinton said the US government had noted an increase in hacking attacks on both state institutions and private companies, and would raise the issue in international forums.
"We have to begin making it clear to not only the Chinese... that the United States is going to be having to take actions to protect not only our governments but our private sector from this kind of illegal intrusion," she said.
"I'd like to see an international forum committed to discussing what to do about this, because everybody's vulnerable.
"We're going to try to get legislation passed, which we were unsuccessful in doing in the last Congress," she told journalists at a briefing to mark the end of her term as America's top diplomat.
The Journal gave no timeline for the attacks but said a network overhaul to bolster security had been completed on Thursday.
"We fully intend to continue the aggressive and independent journalism for which we are known," Dow Jones spokeswoman Paula Keve said.
On Wednesday, the Times reported that hackers had infiltrated computer systems and stolen staff passwords over the past four months.
The effort was particularly focused on the emails of Shanghai bureau chief David Barboza, the newspaper said.
Barboza wrote a story, published October 25, that said close relatives of Chinese Premier Wen Jiabao had made billions of dollars in business dealings.
"Chinese hackers, using methods that some consultants have associated with the Chinese military in the past, breached The Times's network," the newspaper said, citing a wealth of digital evidence gathered by its security experts.
The Times said the IT consultants believed the attacks "started from the same university computers used by the Chinese military to attack United States military contractors in the past."
Asked for comment on the allegations involving the Wall Street Journal, China's defense ministry referred AFP to a statement it made Thursday saying that the military had "never supported any hacking attacks."
"Cyber-attacks have a transnational and anonymous nature; under such circumstances accusing the Chinese military of launching attacks through the web without irrefutable proof is unprofessional and baseless," it said.
The Times said hackers stole corporate passwords and targeted the computers of 53 employees, including former Beijing bureau chief Jim Yardley, who is now the Times's South Asia bureau chief based in India.
The newspaper said Bloomberg News was also targeted by Chinese hackers, after publishing a report in June on the wealth accumulated by relatives of Xi Jinping. In November, Xi was elevated to leader of the Chinese Communist Party.
The Beijing correspondent of Canada's Globe and Mail newspaper, Mark MacKinnon, meanwhile said he had been hacked in 2011. "Computer expert told me hack targeted at specific China-related files," he wrote on Twitter.