The US ambassador to Beijing, Nicholas Burns, was reportedly one of the American officials whose emails were accessed in a recent Chinese hacking attack which took Washington by surprise with its sophistication. Another target was Daniel Kritenbrink, the assistant secretary of state for east Asia, the Wall Street Journal reported on Thursday. When the attack was first disclosed last week, the administration admitted the email account of the commerce secretary, Gina Raimondo, had also been compromised. US officials were quoted as saying those were the three most senior targets but that in total, hundreds of thousands of government email accounts could have been breached. Asked for comment on the report, a state department spokesperson said: “For security reasons, we will not be sharing additional information on the nature and scope of this cybersecurity incident at this time.” “The department continuously monitors and responds to activity of concern on our networks,” the spokesperson said. “Our investigation is ongoing, and we cannot provide further details at this time.” It was not clear how much sensitive US government information had been compromised. According to the Journal’s account, Secretary of State Antony Blinken’s email had not been breached, nor had those of his inner circle of advisers. But Kritenbrink accompanied the secretary on his visit to China last month, and Burns had also attended meetings with the country’s leader, Xi Jinping, and other senior Chinese officials in the course of the visit. It is possible the hackers gained access to US preparations for those meetings and internal discussions about them. US intelligence officials are reported to have been taken by surprise by the stealth and sophistication of the cyber-attack, which exploited a flaw in Microsoft’s cloud computing environment which has since been fixed, the company said. Microsoft identified the perpetrators as the Chinese group Storm-0558, which it called “well-resourced” and “focused on espionage”.