The New York Times reports that Russian hackers who tapped into the White House computer system last year were able to read President Barack Obama's unclassified e-mail.
The newspaper quoted unnamed U.S. officials in its April 25 report.
A White House spokeswoman declined to comment on the report but the White House earlier this month confirmed the breach, saying it took place last year and that it did not affect classified information.
The United States has also not confirmed reports that Russia was behind the attack.
The New York Times reported that the breach had been "far more intrusive and worrisome than has been publicly acknowledged."
It said that although no classified networks had been compromised, officials conceded that the unclassified system still contains highly sensitive information such as e-mail exchanges with diplomats and exchanges about personnel moves and legislation.
Based on reporting by The New York Times, Reuters, and AFP