In May, hackers linked to the Chinese state accessed the email accounts of about 25 organizations, including those of the US government, in a covert cyber espionage campaign, Microsoft and US officials announced on Wednesday.

The United States discovered the intrusions into federal government accounts “quite quickly” and managed to prevent further intrusions, White House National Security Adviser Jake Sullivan said in an interview with the American television network ABC.

The US State Department was one of the government agencies affected, according to a person familiar with the investigation, who spoke on condition of anonymity.

Microsoft identified the hacker group as Storm-0558.

“Microsoft has contacted all organizations that were targeted by the attack or those that were directly compromised and provided them with important information to help them investigate the matter and respond,” the company said in a statement.

Microsoft did not say which organizations or governments were affected, but added that the group of hackers had targeted entities in Western Europe.

The Chinese embassy in London called the accusation “disinformation” and the US government “the world’s largest hacker empire and global cyber thief”, as its officials put it. China has consistently denied involvement in hacking operations, despite the available evidence.

White House National Security Council spokesman Adam Hodge said an intrusion into Microsoft’s cloud security system had “affected unclassified systems,” but gave no other details.

The State Department announced that it had taken immediate steps to secure our systems following the detected intrusion.

Private-sector cybersecurity experts say recently discovered hacking activities show Chinese groups are improving their cyber capabilities.