Attackers could black out entire US: Report

Cars are blurred as they pass by a darkened Flatiron Building in a section of Manhattan still in a blackout following Hurricane Sandy on October 30, 2012 in New York City.

A study by the Federal Energy Regulatory Commission raises concerns about the seriousness of electricity grid vulnerability, The Wall Street Journal reports.

The study found that if saboteurs attacked just nine of the country's 55,000 electric-transmission substations, the country's power network would collapse. An ensuing nationwide blackout could last weeks or even months, the newspaper said.

(Read more: US power grid system to undergo simulated attack)

Protecting the nation's power grid from threats

"This would be an event of unprecedented proportions," Ross Baldick, a professor of electrical engineering at the University of Texas at Austin, told the Journal.

At the moment, there are no federal rules requiring utilities to protect substations, the newspaper said.

In January, M. Granger Morgan, head of engineering and public policy at Carnegie Mellon University, told CNBC that a physical attack on the grid would be a "much greater threat" than a cyberattack.

Read the Journal's full story here.