The U.S. has uncovered evidence that cyberspies, most likely from China and Russia, have penetrated the U.S. power grid and left behind software that could be activated to disrupt American infrastructure, FOX News confirmed Wednesday. 

The "intrusions," first reported by The Wall Street Journal, have occurred over a period of time, one U.S. official said -- not all at once. 

The breaches are "something we're concerned about," a U.S. official told FOX News. 

The concern is that any software could be activated at a later date to disrupt critical systems. 

The intrusions were not just limited to the electrical power grid, but affected systems like water and sewage. 

White House spokesman Nick Shapiro said in a written statement that President Obama takes the issue of cyber security "very seriously," but that the administration is not aware of any "disruptions" to the power grid. 

"Cyber attacks are made all the time, however, industry continuously looks for and mitigates against such attacks and we are not aware of any disruptions to the power grid caused by deliberate cyber activity here in the United States," he said. 

Homeland Security Secretary Janet Napolitano said the same, but would not comment on the apparent "intrusions" committed by cyberspies. She said the government has known about the vulnerability of the system for years. 

"Nonetheless, we remain in constant protection, prevention, education, resiliency mode and we work with the utility sector in particular on that," she said. 

The motivation for the breaches is not well understood, and while the electronic trail appears to lead to China and Russia, it is not clear whether these actions were state-sponsored. 

The Washington embassies of China and Russia deny involvement. 

According to The Wall Street Journal, the espionage appeared pervasive across the country and did not target any particular region or company. 

The intrusions were in many cases detected by U.S. intelligence agencies, not the companies, officials told the Journal. 

"If we go to war with them, they will try to turn them on," one official told the Journal. 

Click here to read the story in The Wall Street Journal. 

FOX News' Catherine Herridge, Mike Levine, Mike Emanuel and Mosheh Oinounou contributed to this report.