Emails to and from United States 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.
In October last year, the Obama administration revealed that an unclassified computer network used by the US government had been infiltrated by hackers. While US officials admitted there was a cyber "event", they refused to confirm reports that Russia was behind the attack.
The security breach caused temporary system outages "as a result of measures we have taken to defend our network", according to an unnamed official quoted in The New York Times at the time, as the unusual network activity was spotted and dealt with.
However, the incursion was "far more intrusive and worrisome" than publicly acknowledged, The New York Times said in a report on Saturday, citing senior American officials briefed on the investigation. It said 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, reportedly obtained access to the email archives of people inside the White House, and perhaps some outside, with whom Obama regularly communicated.
It was from those accounts that they reached emails 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.
However, The New York Times reported that officials conceded that the unclassified system often contains information considered highly sensitive, such as schedules, email exchanges with "ambassadors and diplomats, discussions of pending personnel moves and legislation, and, inevitably, some debate about policy".
It should be noted that Obama's email account itself did not appear to have been hacked, although it is not clear how many of the president's emails were read, the report said.
"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.
October's White House network breach came as online security company FireEye released a detailed report suggesting that state-sponsored attacks originating from Russia had focused on lifting military, government, and security information.
In October, FireEye released a report (PDF) detailing the activities of APT28, a Russian hacking group that has been in operation since 2007, believed to have targeted US defence and military contractors, NATO officials, and others such as the Republic of Georgia and European security firms.
FireEye said at the time that "skilled" Russian developers and operators could be linked to APT28 through a government sponsor based in Moscow.
The emergence of further details surrounding October's breach comes just over a month after another digital heavyweight, China, made an admission to the existence of special cyberwarfare units.
China's digital war tactics were discussed in the latest edition of publication The Science of Military Strategy, which is produced by China's People's Liberation Army (PLA).
US Center for Intelligence Research and Analysis researcher Joe McReynolds told The Daily Beast at the time that, "this is the first time we've seen an explicit acknowledgement of the existence of China's secretive cyberwarfare forces from the Chinese side".
Tension between China and the US has escalated after a series of cyber espionage accusations between the two countries -- triggered, to a large extent, by the data-gathering activities undertaken by the US National Security Agency's Prism program, made public by whistle-blower Edward Snowden.