Washington: Chinese and Russian hackers are attempting to seed viruses in the US power grid that could one day plunge major cities into chaos, a report warned Wednesday.

The report in the Wall Street Journal quotes intelligence officials saying that cyber-spies last year repeatedly gained access to the system powering everything from financial institutions to sewage systems.

“The Chinese have attempted to map our infrastructure, such as the electrical grid,” the daily said quoting an unidentified senior intelligence official, and “so have the Russians.”

While no damage was caused, investigators found time bomb style viruses sown into the system. “If we go to war with them, they will try to turn them on,” the official was quoted as saying.

Amy Kudwa, spokeswoman for the Department of Homeland Security, said she was “not aware of any disruption to the power grid.”

However, she acknowledged that “cyber attacks are made all the time.”

Barack Obama, the most Internet-savvy president in US history, has pledged to expand the use of the Internet across the country. Among the projects is a major overhaul, with increasing reliance on computer networks, of the electric grid.

That adds to pressure to defend against the growing army of cyber assailants.

On Tuesday the Pentagon revealed that more than 100 million dollars had been spent just in the last six months to repair damage from attacks.

Kudwa said that DHS Secretary Janet Napolitano “takes the issue of cyber security very seriously, which is why she has ordered a top-to-bottom review of all our cyber security.”

The DHS and private industry “continuously looks for and mitigates those attacks,” she said.

But the challenge is spiraling.

“We’re spending vast amounts of money trying to improve security, but computer security is a moving target. It’s a journey without destination,” said John Bumgarner at Cyber Consequences Unit, which advises the government.

Evan Kohlmann, an investigator with Global Terror Alert, said cyber assailants were capable of replicating the accidental power outage briefly shutting down New York in 2003.

That incident resulted from a fault in power lines, but a hacker controlling the grid could inflict similar or even worse disruption.

“That was just the power going off briefly. Imagine if worse things started to happen. If you induced power surges you could cause very, very serious permanent damage,” Kohlmann said. “You could cause mass economic damage.”

Although terrorist groups might consider such targets, the most skilled and motivated hackers are in China and Russia, analysts say.

“Both (China and Russia) are particularly interested in enlisting their populace. The Chinese government has either allowed to flourish or has encouraged many patriotic hackers and the Russian government too,” said Noah Shachtman, an editor at Wired magazine.

Bumgarner, a government special operations veteran and hacking expert, said that many attacks on the grid aim not to cause damage, but to steal information.

“Some could be just to extract data to increase the efficiency of their own systems,” he said.

Kohlmann said countries like China and Russia, which are rivals but also partners to the United States, are not interested in causing major damage — at least now.

“It appears their aim is not to disrupt the systems now, but to ensure that if these states were ever in a position where they have their backs against the wall that they have another option to atomic weapons or whatever.”

The more immediate threat is that the hacking expertise gets out from under government control.

“Once you have the genie out of the bottle and people able to do this, soon it won’t be a team of people in a government lab,” Kohlmann said. “It’s really only a matter of time before non-state actors can get in.”

Bumgarner said that cyberspace has become a fully fledged front in national security, along with air, land, sea and space.

“The United States’ digital footprint touches all across the world, just as theirs touches ours.”