The US State Department says about 60,000 emails were stolen from the department in a cyberattack reportedly perpetrated by a China-based hacker group.
Spokesperson Matthew Miller revealed this at a news conference on Thursday. He said classified systems were not hacked.
Microsoft announced in July that a China-based hacking group had breached email accounts linked to about 25 government agencies and organizations.
The State Department confirmed the breach and has been investigating the case.
US media outlets say 10 State Department email accounts were hacked. They note that nine of them belonged to officials working on East Asian and Pacific Affairs.
A reporter asked Miller if the State Department has independently determined who was behind the attack. Miller responded, "We have no reason to doubt the attribution that Microsoft has made publicly."
The US government is on high alert for cyberattacks from China. It says China routinely conducts malicious cyber activities as a means of trying to counter US military power.