First, The New York Times said its system had been attacked by Chinese hackers.
Now, the Wall Street Journal is the second major US media outlet to make the same accusation, saying hackers are trying to spy on journalists reporting on China.
Its claims come a day after The New York Times said Chinese hackers, possibly connected to the military, had infiltrated its computers and stolen staff passwords over in response to an expose of the vast wealth amassed by the family of China's Premier, Wen Jiabao.
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.
The Journal gave no timeline for the attacks, but said a network overhaul had been completed on Thursday, to bolster security.
"We fully intend to continue the aggressive and independent journalism for which we are known," Dow Jones spokeswoman Paula Keve said.
Asked for comment on the allegations involving the Wall Street Journal, China's Defence 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 outgoing US 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.