Foreign spies have infiltrated the US electrical grid, leaving behind software programs that could disrupt the system in a time of war, American national security officials have claimed.

The intruders, who came from countries including China and Russia, were believed to be attempting to map the US electrical system and work out how it was controlled, according to reports in the Wall Street Journal.

Officials said the cyberspies had not tried to damage the grid, but warned they could during a crisis or war.

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

The intrusion spread across the country and didn't target any specific companies or regions, a former Department of Homeland Security official said. "There are intrusions, and they are growing," the former official said, referring to electrical systems. "There were a lot last year."

Several of the intrusions were detected by US intelligence agencies and not by the companies in charge of the infrastructure, the officials said.

The breaches come as concern grows among the intelligence community over cyber attackers taking control of electrical facilities, a nuclear power plant or financial networks via the internet.

More worrying was the discovery that the cyberspies had left behind software tools that could be used to destroy infrastructure components, the senior intelligence official said. He told the Wall Street Journal: "If we go to war with them, they will try to turn them on."

Water, sewage and other infrastructure systems were also believed to be at risk.

"Over the past several years, we have seen cyberattacks against critical infrastructures abroad, and many of our own infrastructures are as vulnerable as their foreign counterparts," Director of National Intelligence Dennis Blair recently told politicians. "A number of nations, including Russia and China, can disrupt elements of the U.S. information infrastructure."