Beijing-linked hackers accessed the email account of the US ambassador to China in an espionage operation thought to have compromised at least hundreds of thousands of US government emails, the Wall Street Journal (WSJ) reported on Thursday.

Daniel Kritenbrink, the assistant secretary of state for East Asia, was also hacked in the wider spying operation disclosed this month by Microsoft, the newspaper 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 going on.

US Ambassador Nicholas Burns’ embassy in Beijing referred Reuters to remarks made by Secretary of State Antony Blinken this month when he said the US has “consistently made clear to China as well as to other countries that any action that targets the US Government or US companies, American citizens, is of deep concern to us, and we will take appropriate action in response.”

A spokesperson for China’s embassy in Washington said China consistently opposed hacking and it rejected “groundless” speculation about the source of cyber attacks.

“China firmly opposes and combats cyber attacks and cyber theft in all forms. This position is consistent and clear,” spokesperson Liu Pengyu said in an emailed response to Reuters.

“Identifying the source of cyber attacks is a complex technical issue. We hope that relevant sides will adopt a professional and responsible attitude … rather than make groundless speculations and allegations.”

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

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