The news company, whose publications and businesses include the New York Post and WSJ parent Dow Jones, said it discovered the breach on Jan. 20. It said in a regulatory filing that an investigation is underway “to determine its nature, scope, duration and impacts.” It said customer and financial data were so far not affected and that the company’s operations had not been interrupted.

But a major concern was the company’s journalists. News organizations are prime targets for the world’s intelligence agencies because their reporters are in constant contact with sources of sensitive information. Journalists and newsrooms from Mexico and El Salvador to the United Arab Emirates have been hacked with powerful spyware.

Mandiant, the cybersecurity firm examining the hack, said in a statement that it “assesses that those behind this activity have a China nexus, and we believe they are likely involved in espionage activities to collect intelligence to benefit China’s interests.”

It was not known when the hackers breached the network or how much data they stole.

In an email to staff, News Corp said the hack “affected a limited number” of email accounts and documents from News Corp headquarters, News Technology Services, Dow Jones, News UK, and New York Post.

“Our preliminary analysis indicates that foreign government involvement may be associated with this activity, and that some data was taken,” the email said.

“Our highest concern is the protection of our employees, including our journalists, and their sources,” it added, saying it believed the “threat activity is contained.”

FBI Director Christopher Wray said in a speech this week that the bureau opens investigations tied to suspected Chinese espionage operations about every 12 hours, and has more than 2,000 such probes. He said Chinese government hackers have been pilfering more personal and corporate data than all other countries combined.

While state-backed Russian hacking tends to get more headlines, U.S. officials say China has been stealthily stealing far more valuable commercial and personal data over the past few decades as digital technology took hold.

Major newsrooms, including The New York Times, against which a Chinese cyberespionage operation was uncovered in 2013, have previously been compromised.

Runa Sandvik, former director of information security at the newspaper, said that while major newsrooms have shown a lot of progress in the last few years in helping their journalists navigate an increasingly hostile digital world, those efforts are not adequate to defend against a skilled and determined adversary like China.

A spokesperson for the Chinese Embassy in Washington did not immediately return an email seeking comment.

News Corp.’s assets also includes the publishing house HarperCollins, News Corp Australia and Storyful, which the email to employees said were apparently not targeted by the hackers.

—Bajak reported from Boston. Associated Press writer David Bauder in New York contributed to this report.