The US National Security Agency has developed a secret technology to spy on computers via radio signals, gathering information even when the devices are offline, a report said Wednesday.

The New York Times reported that the NSA has implanted software on 100,000 computers around the world to be able to conduct surveillance, and which gives the spy agency "a digital highway" for launching cyberattacks.

The Times, citing unnamed sources, said the agency has used the program code-named Quantum since at least 2008, relying on a covert channel of radio waves that can be transmitted from tiny circuit boards and cards inserted surreptitiously into the computers.

It said the radio technology has helped solve a key problem for US intelligence agencies, by getting into computers of adversaries that are hardened against attacks. But it noted that the radio frequency hardware must in most cases be physically inserted by a spy, a manufacturer or an unwitting user.

The report said the NSA and its Pentagon partner, United States Cyber Command, have used these techniques against the Chinese Army, which has been accused of cyberattacks on US firms.

It has also been used against Russian military networks, Mexican police and drug cartels, trade institutions inside the European Union, and sometime partners against terrorism like Saudi Arabia, India and Pakistan, according to officials and documents cited by the US newspaper.

In a statement to AFP, the NSA did not directly comment on the report but said the agency "deploys various foreign intelligence techniques to help defend the nation."

"As we have previously stated, the implication that NSA's collection is arbitrary and unconstrained is false. NSA's activities are focused and specifically deployed against -- and only against -- valid foreign intelligence targets in response to intelligence requirements," the statement said.

"In addition, we do not use foreign intelligence capabilities to steal the trade secrets of foreign companies on behalf of -- or give intelligence we collect to -- US companies to enhance their international competitiveness or increase their bottom line."

White House spokesman Jay Carney, asked about the report, echoed the NSA comments.

"I won't discuss specific tools. But the NSA operates under heavy oversight and is focused on discovering and developing intelligence about valid foreign intelligence targets such as terrorists, human traffickers and drug smugglers," Carney told reporters on Air Force One.

"They are not interested in the personal information about ordinary Americans."

President Barack Obama is due to unveil proposals Friday to reform US intelligence in the wake of a series of revelations which have stunned US allies and others on the vast data-gathering capabilities of the spy agencies, based on leaked documents from former NSA contractor Edward Snowden.