Beijing-linked hackers accessed U.S. Ambassador to China Nicholas Burns' email account in an espionage operation thought to have compromised at least hundreds of thousands of individual U.S. government emails, the Wall Street Journal reported on Thursday.

Daniel Kritenbrink, the assistant secretary of state for East Asia, was also hacked in the wider spying operation disclosed earlier this month by Microsoft, the report said, citing people familiar with the matter.

Asked about the reported breach of the two diplomats' accounts, the State Department declined to give any details and said its investigation of the spying operation was ongoing, adding: "For security reasons, we will not be sharing additional information on the nature and scope of this cybersecurity incident at this time."

Burns and Kritenbrink join U.S. Commerce Secretary Gina Raimondo as the only publicly named victims of the espionage campaign, which prompted a warning by Washington's top diplomat to his Chinese counterpart.

Microsoft said last week that Chinese hackers misappropriated one of its digital keys and used a flaw in its code to steal emails from U.S. government agencies and other clients.

The company did not immediately return a message seeking comment on the WSJ report.

China's embassy in Washington did not immediately respond to a request for comment on the report, but the Chinese Ministry of Foreign Affairs previously called the earlier accusations "disinformation."