China-linked hackers breached the email account of U.S. Ambassador to China Nicholas Burns, as part of a recent targeted intelligence-gathering campaign, NBC News has confirmed.

The hackers also accessed the email account of Daniel Kritenbrink, the assistant Secretary of State for East Asia, who recently travelled with Secretary of State Antony Blinken to China, said NBC, citing two U.S. officials familiar with the matter. 

CNBC reached out to China’s Foreign Ministry for comment but has yet to hear back.

The beach was limited to the diplomats’ unclassified email accounts, NBC said adding that Secretary of Commerce Gina Raimondo’s email account was also accessed in the breach, as previously reported.

The news, first reported by the Wall Street Journal, further fuels the fallout for the U.S. of the alleged Chinese hack first revealed last week. 

Late Tuesday, Microsoft announced it had discovered that China-based hackers breached email accounts of about 25 organizations, including some U.S. government agencies, in a significant breach.

The compromise was “mitigated” by Microsoft cybersecurity teams after it was first reported to the company in mid-June 2023, Microsoft said in two blog posts about the incidents. The hackers had been inside government systems since at least May, the company said.

Blinken said he raised the issue of the Chinese hacking when he met China’s top diplomat Wang Yi in Jakarta last week, on the sidelines of the Association of Southeast Asian Nations regional meeting.

The U.S. Secretary noted he made clear to Wang that Washington will ensure the hackers are held responsible for alleged breaches of U.S. government agencies.

“First of all, this is something that the State Department actually detected last month, and we took immediate steps to protect our systems, to report the incident – in this case, notifying a company, Microsoft, of the event,” Blinken said at a press briefing.

“I can’t discuss details of our response beyond that, and most critically this incident remains under investigation,” he added.

Still, Blinken said that as a general matter, “we have consistently made clear to China as well as to other countries that any action that targets the U.S. Government or U.S. companies, American citizens, is of deep concern to us, and we will take appropriate action in response.”

The secretary’s latest meeting with Wang came less than a month after Blinken made a rare trip to Beijing under the Biden administration.

The visit was aimed at soothing ties between the world’s two largest economies amid escalating tensions.

Security experts have argued the incidents demonstrate an acceleration in Beijing’s digital spying capabilities.

“Chinese cyber espionage operators’ tactics had steadily evolved to become more agile, stealthier, and complex to attribute” over the last decade, researchers at cybersecurity firm Mandiant wrote in a blog post Tuesday.

— CNBC’s Rohan Goswami contributed to this report.