Emails to and from President Barack Obama were read by Russian hackers last year in a breach of the White House's unclassified computer system, according to The New York Times.
Earlier this month, US officials admitted there was a cyber "event" late last year, but refused to confirm reports Russia was behind the attack.
However the incursion was "far more intrusive and worrisome" than publicly acknowledged, the newspaper said, citing senior American officials briefed on the investigation, and saying the hackers were presumed to be linked to -- or even working for -- Moscow.
The hackers, who also got into the State Department's unclassified system, obtained access to the email archives of people inside the White House, and perhaps some outside, with whom Obama regularly communicated, the Times said.
It was from those accounts that they reached emails that the president had sent and received, according to the officials.
The hackers did not appear to have penetrated the servers that control the message traffic from Obama's BlackBerry and the White House has said that no classified networks were compromised.
"But officials have conceded that the unclassified system routinely contains much information that is considered highly sensitive: schedules, email exchanges with ambassadors and diplomats, discussions of pending personnel moves and legislation, and, inevitably, some debate about policy," the Times reported.
It was not clear how many of Obama's emails were read, the Times said, adding that his email account itself did not appear to have been hacked.
"Still, the fact that Mr Obama's communications were among those hit by the hackers -- who are presumed to be linked to the Russian government, if not working for it -- has been one of the most closely held findings of the inquiry," the newspaper said.
"It's the Russian angle to this that's particularly worrisome," a senior official was quoted as saying.
Most of Obama's classified briefings are delivered orally or on paper and are usually confined to the Oval Office or the Situation Room, the Times said.