A computer security unit within the US Federal Bureau of Investigation (FBI) has detected a series of intrusions into US government networks under an investigation code named Moonlight Maze.
An FBI official told US Congress last week the intrusions appeared to have originated from Russia.
The intrusions accessed government, university and private sector networks and took large amounts of unclassified but sensitive information, including defence technology research information, said Michael Vatis, director of the National Infrastructure Protection Center (NIPC), in testimony before the Senate Judiciary Committee's subcommittee on technology, terrorism and government information.
Vatis said the intrusions took place during the past few years, but he declined to elaborate because the Moonlight Maze investigation is ongoing.
A spokesman for the Russian embassy quoted the head of the press service for the Russian foreign intelligence service, Boris Labusov, as saying the Russian special services have "no relation whatsoever" to the theft of information from computer networks of the US federal agencies.
"American specialists have failed to establish from where this intrusion originated," the embassy official quoted Labusov as saying in an interview with the Russian news agency Itar-Tass.
"They only indicated that it comes from the direction of Moscow. Russian special services are not so stupid to undertake such an operation, in case the necessity arises, directly from Moscow."
Ordinary hackers, or possibly the intelligence services of third countries, may be using Moscow to conceal their real address, Labusov said.
NIPC investigators, who have been coordinating with FBI field offices, the Department of Defence and other government agencies, are concerned the intrusions are a sign that foreign militaries are exploring the prospects of a new way to wage war that has little to do with the traditional battlefield.
"We know that several foreign nations are already developing information warfare doctrines, programs and capabilities," Vatis said in written testimony submitted to the subcommittee.
"They see that they cannot defeat the United States in a head-to-head military encounter and they believe that information operations are a way to strike at what they perceive as America's Achilles Heel: our reliance on information technology to control critical government and private sector systems."
Vatis stressed the importance that Congress and the American public understand the "very real threat that we are facing in the cyber realm, not just in the future, but now."