Hackers responsible for breaching the White House's IT systems managed to read President Barack Obama's personal emails, it is claimed.
The cyber criminals who broke into the White House's network last October got their hands on some of Obama's messages via the accounts of his correspondents, according to the New York Times.
The publication cited anonymous US officials in a report that called the attack "far more intrusive and worrisome than has been publicly acknowledged".
Russia is thought to be behind the hack, which breached the unclassified network in the White House, but not the classified system.
However, the NYT claims far more sensitive information was shared over the unclassified system than previously thought.
"This has been one of the most sophisticated [threat] actors we've seen," one official told the US newspaper.
Another said: "It's the Russian angle to this that's particularly worrisome."
While the cyber criminals are understood to have not accessed Obama's own email account, which sits on highly secure servers, they did manage to access the accounts of White House officials with whom he communicated.
By scrolling through their email archives, the hackers discovered Obama's messages, the NYT claims.
A spokeswoman for the National Security Council declined to comment on the report.
The revelation comes hot on the heels of US secretary of defence, Ashton Carter, confirming Russian hackers had targeted the Pentagon's unclassified systems.
It's not known whether the two attacks are linked.
Obama announced his intention to use tougher measures to deter foreign hackers at the start of April.
"The increasing prevalence and severity of malicious cyber-enabled activities originating from, or directed by, persons located, in whole or in substantial part, outside the US constitute an unusual and extraordinary threat to the national security, foreign policy and economy of the US. I hereby declare a national emergency to deal with this threat," the President announced.
The sanctions include limiting foreign countries' access to American financial systems and technology.
