Russian hackers have been able to penetrate the White House's computer system and see non-classified emails, CNN reports.

The hack didn't get access to classified materials, government sources told CNN, but still was able to see sensitive information such as real-time nonpublic details of the president's schedule.

Hackers first managed to hack into State Department computers and tricked an employee into providing information that gave them a back door into the White House system and eventually the Oval Office. Hackers have tried various tricks to get into US government systems over the years, including dropping thumb drives in parking lots and government restrooms, where unsuspecting personnel pick them up and put them in home and work computers.

Shawn Henry, former executive assistant director of the FBI, told CNN's "Anderson Cooper 360" that the hackers might have been working for the Russian government, but also could have been sympathizers who turned over their information to the Russian government.

They also could have been independent hackers being monitored by the Russian government, which then benefited from the information.

"With that level of access that they've got, they've got the ability to see everything that occurs on that network, they've got the ability to exfiltrate data off that network and most probably most important, they've got the ability to destroy components off that network," Henry said.

The government has not officially admitted the hack occurred, CNN said, but Director of National Intelligence James Clapper recently hinted at the attack during testimony to the Senate.

"While I can't go into detail here, the Russian cyber threat is more severe than we had previously assessed," Clapper testified.

Appearing on CNN's "The Situation Room with Wolf Blitzer," Deputy National Security Adviser Ben Rhodes declined to give details on the attack.

"What I can say, though, is … we were public about the fact that we were dealing with cyber intrusions in the State Department," he said.

Rhodes said there are different systems in the White House for classified and non-classified information, and that the system that handles top-secret information was not compromised.

The White House constantly upgrades its non-classified system, Rhodes said, but users are told not to put sensitive information on that system.

"In other words, if you're going to do something classified, you have to do it on one email system, one phone system," he said. "Frankly, you have to act as if information could be compromised if it's not on the classified system."

Russia hackers have been getting into the State Department system for months, despite constant efforts to keep them out, CNN previously reported.

Rhodes also declined to say whether Russia is believed to be behind the hack.

"Russia has been certainly active in the cyberspace and active in the espionage space. We're always taking precautions against that cyber danger," he told CNN.

Other countries, including Iran, also are active in cyberspace, he said.

"This report is not referring to a new incident - it is speculating on the attribution of the activity of concern on the unclassified ... network that the White House disclosed last year," White House spokesman Mark Stroh said in a statement. 

"Any such activity is something we take very seriously. In this case, as we made clear at the time, we took immediate measures to evaluate and mitigate the activity. As has been our position, we are not going to comment on the referenced article's attribution to specific actors."

The U.S. said last year it suspected the Russian government or criminal hackers of being behind the October attack on a White House computer system.

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 e-mail to communicate with colleagues, according to U.S. officials who spoke on condition of anonymity.

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