Barack Obama,

President Barack Obama reads as he walks back to the Oval Office of the White House in Washington, Tuesday, April 7, 2015. Russian-hired hackers apparently breached an unclassified White House system and had access to information about President Obama's daily schedule and communications.

(Pablo Martinez Monsivais, Associated Press)

Russian-hired hackers apparently breached an unclassified White House system and had access to information about President Barack Obama's daily schedule and communications.

Russian hackers behind the damaging cyber intrusion of the State Department in recent months used that perch to penetrate sensitive parts of the White House computer system, according to U.S. officials briefed on the investigation, CNN News is reporting.

While the White House has said the breach only ever affected an unclassified system, that description belies the seriousness of the intrusion, according to CNN. The hackers had access to sensitive information such as real-time non-public details of the president's schedule. While such information is not classified, it is still highly sensitive and prized by foreign intelligence agencies, U.S. officials told CNN.

A phishing-style attack allowed Russian hackers to access a State Department computer network, which in turn allowed access to the White House's system, Business Insider reported. The hackers were working for the Russian government, but did not succeed in accessing any classified networks, according to CNN.

Last October, the White House said it noticed suspicious activity in the unclassified network that serves the executive office of the president. The system has been shut down periodically to allow for security upgrades, CNN reported.

The FBI, Secret Service and U.S. intelligence agencies are all involved in investigating the breach, which they consider among the most sophisticated attacks ever launched against U.S. government systems, according to CNN. The intrusion was routed through computers around the world, but investigators found codes and other markers they believe point to hackers working for the Russian government.

Some U.S. officials investigating the intrusion suspect that the hackers may have gotten into unclassified White House systems after gaining entry at the State Department, where officials regularly use email to communicate with colleagues, according to U.S. officials who spoke on condition of anonymity, Bloomberg News reported.

The State Department incident late last year proved difficult to root out, the officials said, according to Bloomberg News, because the hackers kept revising their tools to foil defensive efforts.

Ben Rhodes, President Obama's deputy national security adviser, told CNN the White House's use of a separate system for classified information protected sensitive national security-related items from being obtained by hackers.

"We do not believe that our classified systems were compromised," Rhodes told CNN Tuesday.

"We're constantly updating our security measures on our unclassified system but we're frankly told to act as if we need not put information that's sensitive on that system," he told CNN. "In other words, if you're going to do something classified, you have to do it on one email system, one phone system. Frankly, you have to act as if information could be compromised if it's not on the classified system."

Non-classified networks can contain sensitive information the White House might not necessarily want in the hands of a rival government. And it's a sign that even high-level U.S. government systems are far from impervious to outside attacks, Business Insider reported.