The US State Department on Wednesday said it had no reason to doubt Microsoft's accusation that Chinese state-linked hackers have since May secretly accessed email accounts at about 25 organisations, including US government agencies, in an espionage campaign.
State Department spokesman Matthew Miller said the agency detected "anomalous activity" in June and immediately notified Microsoft.
“We took immediate steps to secure our system and took immediate steps to notify Microsoft of the event,” Mr Miller told reporters.
The US has not publicly attributed the attack to anyone but Microsoft has said it “mitigated a China-based threat” on behalf of a customer.
Microsoft said hackers in China, from May 15, "gained access to email accounts affecting approximately 25 organisations, including government agencies, as well as related consumer accounts of individuals likely associated with these organisations", the tech giant said.
A timeline shared by Microsoft said the attempted breach was discovered just before Secretary of State Antony Blinken travelled to China, where he met Chinese President Xi Jinping and other leaders.
The breach has raised concern among US politicians over just how secure government agencies are.
“The Senate intelligence committee is closely monitoring what appears to be a significant cyber-security breach by Chinese intelligence,” said Mark Warner, chairman of the panel.
“It’s clear that the PRC [People's Republic of China] is steadily improving its cyber-collection capabilities directed against the US and our allies.”