The spies came from China, Russia and other countries, and were believed to be on a mission to navigate the U.S. electrical system and its controls, the newspaper said, citing current and former U.S. national security officials.

The intruders have not sought to damage the power grid or other key infrastructure but officials said they could try during a crisis or war, the paper said in a report on its website.

"The Chinese have attempted to map our infrastructure, such as the electrical grid," a senior intelligence official told the Journal. "So have the Russians."

The espionage appeared pervasive across the United States and does not target a particular company or region, said a former Department of Homeland Security official.

"There are intrusions, and they are growing," the former official told the paper, referring to electrical systems. "There were a lot last year."

The administration of U.S. President Barack Obama was not immediately available for comment on the newspaper report.

Authorities investigating the intrusions have found software tools left behind that could be used to destroy infrastructure components, the senior intelligence official said. He added, "If we go to war with them, they will try to turn them on."

Officials said water, sewage and other infrastructure systems also were at risk.

Protecting the electrical grid and other infrastructure is a key part of the Obama administration's cybersecurity review, which is to be completed next week.

The sophistication of the U.S. intrusions, which extend beyond electric to other key infrastructure systems, suggests that China and Russia are mainly responsible, according to intelligence officials and cybersecurity specialists.

While terrorist groups could develop the ability to penetrate U.S. infrastructure, they do not appear to have yet mounted attacks, these officials say.