Senior American officials say Russian hackers read some of President Barack Obama's unclassified emails last year in a breach of the White House's computer system, The New York Times reports.

The hackers may have gotten access to the email archives of people inside the White House who Obama communicated with regularly. Officials briefed an investigation and found that the hackers were able to reach emails that the president sent and received.

Yet, Russian hackers were not able to access servers that control the messages sent from Obama's BlackBerry that he or an aide carries constantly.

Meanwhile, White House officials say the hackers did not compromise any classified networks, nor did they collect any classified information.

The number of emails hackers did access was not disclosed however. Hackers may have accessed information that contains little debate on policy, discussions on pending personnel moves and legislation.

"This has been one of the most sophisticated actors we've seen," said one senior American official briefed on the investigation.

"It's the Russian angle to this that's particularly worrisome," another senior official said.

Officials had to meet on a daily basis for several weeks after discovering the information about Russian hackers, although the White House says no classified information was compromised in the operation.

The threat with Russian hackers is that they often sweep up information about political targets while the Chinese mainly focus on commercial and design information.

Officials also pointed out that the Russian hackers obtained White House information at the time when new tensions rose between the U.S. and Russia over its annexation of Crimea.

A White House spokeswoman declined to comment on the report but the White House earlier this month confirmed the breach, according to Reuters. The White House apparently knew about the intrusion for months.

George W. Bush gave up emailing during his presidency and refused to carry a smartphone. The White House easily discovers cyberattacks on a daily basis that are not only from Russia and China.