Chinese-based hackers seeking intelligence information breached the email accounts of a number of US government agencies, computer giant Microsoft said.

"The threat actor Microsoft links to this incident is an adversary based in China that Microsoft calls Storm-0558," the company said in a blog post late Tuesday.

Microsoft said Storm-0558 gained access to email accounts at approximately 25 organizations including government agencies.

Microsoft did not identify any of the targets but a US State Department spokesperson said the department had "detected anomalous activity" and had taken "immediate steps to secure our systems."

"As a matter of cybersecurity policy, we do not discuss details of our response and the incident remains under investigation," the spokesperson said.

According to The Washington Post, the breached email accounts were unclassified and "Pentagon, intelligence community and military email accounts did not appear to be affected."

In the blog post, Charlie Bell, a Microsoft executive vice president, said "we assess this adversary is focused on espionage, such as gaining access to email systems for intelligence collection.

"This type of espionage-motivated adversary seeks to abuse credentials and gain access to data residing in sensitive systems," Bell said.

US National Security Adviser Jake Sullivan addressed the hack in an appearance on Wednesday on ABC's Good Morning America, and said it had been detected "fairly rapidly."

"We were able to prevent further breaches," Sullivan said. 

"The matter is still being investigated, so I have to leave it there because we're gathering further information in consultation with Microsoft and we will continue to apprise the public as we learn more," Sullivan said.

Microsoft said Storm-0558 "primarily targets government agencies in Western Europe and focuses on espionage, data theft, and credential access."

The Redmond, Washington-based company said it had launched an investigation into "anomalous mail activity" on June 16.

"Over the next few weeks, our investigation revealed that beginning on May 15, 2023, Storm-0558 gained access to email accounts affecting approximately 25 organizations including government agencies as well as related consumer accounts," it said.

"They did this by using forged authentication tokens to access user email using an acquired Microsoft account consumer signing key," the company said. "Microsoft has completed mitigation of this attack for all customers."

US Senator Mark Warner, chairman of the Senate Select Committee on Intelligence, said the panel is "closely monitoring what appears to be a significant cybersecurity breach by Chinese intelligence."

"It's clear that the PRC is steadily improving its cyber collection capabilities directed against the US and our allies," Warner said in a statement.

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