Washington: The Chinese hackers breached multiple US governments and stole around 60,000 emails from senior State Department officials in May this year, CNN reported citing a Senate staffer.

The new details from a briefing to senators and their staffers illustrate how the Chinese operatives allegedly scoured the inboxes of senior US diplomats focused on diplomacy in the Pacific ahead of a high-stakes trip by US Secretary of State Antony Blinken to China in June.

The hackers raided the unclassified email accounts of nine State Department officials focused on East Asia and the Pacific, and another official who works on European issues, CNN reported citing a staffer in the office of Republican Senator Eric Schmitt of Missouri.

The staffer, who declined to be named, attended a briefing on the hacking campaign that senior State Department IT officials gave the Senate on Wednesday.

The hackers were also able to access a list of every State Department email address, according to the Senate staffer. That kind of reconnaissance could be useful information for any follow-on hacking efforts aimed at the State Department, according to CNN.

At a press briefing on Thursday, State Department spokesperson Matthew Miller confirmed that the hackers accessed about 60,000 State Department emails.

“This was a hack of Microsoft systems that the State Department uncovered and notified Microsoft about,” Miller told reporters.

The State Department has not formally attributed the hack, but Microsoft has blamed a “China-based” hacking group.

“We have no reason to doubt [Microsoft’s] attribution in this case,” CNN quoted Miller as saying.

The stealthy hacking campaign exploited Microsoft email software and began with the hackers breaching a Microsoft engineer, the company has said. The cyber intrusions have showcased the leaps China has made in its cyber capabilities, CNN reported citing experts. This has prompted US lawmakers and Biden administration officials to scrutinize the US government’s reliance on Microsoft technology.

The cyber-espionage campaign breached the unclassified email accounts of US Ambassador to China Nicholas Burns, Commerce Secretary Gina Raimondo, and Daniel Kritenbrink, the assistant secretary of State for East Asia who travelled with Blinken to China in June, CNN previously reported.

Republican Rep. Don Bacon of Nebraska, who has been critical of the Chinese government, said he was also breached by the hackers.

Chinese government officials have responded to the hacking allegations by accusing the US government of conducting cyberattacks against China.

In a statement to CNN, Senator Schmitt welcomed the briefing but said his investigation into the hacks is “far from over.”

“We need to harden our defences against these types of cyberattacks and intrusions in the future, and we need to take a hard look at the federal government’s reliance on a single vendor as a potential weak point,” CNN quoted Schmitt as saying.