Malware attacks from China and Russia designed to shut down the U.S. electrical grid in a time of war did not occur, China said Thursday.

"The incident of attacks on the U.S. electrical grid from China and Russia simply does not exist," Chinese foreign ministry spokeswoman Jiang Yu told reporters, according to a transcript of the briefing.

"We hope the concerned media will cautiously handle groundless statements and especially critiques against China."

Widespread intrusions by cyberspies in countries including China and Russia have infected the U.S. power grid with software that could be used to halt its operation, a Wall Street Journal report said this week, citing unnamed U.S. national security officials.

The newest allegations of Chinese cyberespionage follow long-standing concern that a coordinated attack on the U.S. power grid could cripple its operation.

China produces the majority of the world's malware but part of it could come from attackers in other countries, who often hide behind Chinese IP (Internet Protocol) addresses.