WASHINGTON – The National Security Agency has implanted software in nearly 100,000 computers around the world that allows the United States to conduct surveillance on those machines and can also create a digital highway for launching cyberattacks.

While most of the software is inserted by gaining access to computer networks, the NSA has increasingly made use of a secret technology that enables it to enter and alter data in computers even if they are not connected to the Internet, according to NSA documents, computer experts and U.S. officials.

The technology, which the agency has used since at least 2008, relies on a covert channel of radio waves that can be transmitted from tiny circuit boards and USB cards inserted surreptitiously into the computers. In some cases, they are sent to a briefcase-size relay station that intelligence agencies can set up miles away from the target.

The radio frequency technology has helped solve one of the biggest problems facing U.S. intelligence agencies for years: getting into computers that adversaries, and some U.S. partners, have tried to make impervious to spying or cyberattack. In most cases, the radio frequency hardware must be physically inserted by a spy, a manufacturer or an unwitting user.

The NSA calls its efforts more an act of “active defense” against foreign cyberattacks than an offensive tool. But when Chinese attackers place similar software on the computer systems of U.S. companies or government agencies, U.S. officials have protested, often at the presidential level.

Among the most frequent targets of the NSA and its Pentagon partner, the U.S. Cyber Command, have been units of the Chinese army, which the United States has accused of launching regular digital probes and attacks on U.S. industrial and military targets, usually to steal secrets or intellectual property.

But the program, code-named Quantum, has also been successful in inserting software into Russian military networks and systems used by the Mexican police and drug cartels, trade institutions inside the European Union, and sometime-partners against terrorism such as Saudi Arabia, India and Pakistan, according to officials and an NSA map that indicates sites of “computer network exploitation.”

“What’s new here is the scale and the sophistication of the intelligence agency’s ability to get into computers and networks to which no one has ever had access before,” said James Andrew Lewis, the cybersecurity expert at the Center for Strategic and International Studies in Washington.

“Some of these capabilities have been around for a while, but the combination of learning how to penetrate systems to insert software and learning how to do that using radio frequencies has given the U.S. a window it’s never had before,” he said.

No domestic use seen

There is no evidence that the NSA has implanted its software or used its radio frequency technology inside the United States. While refusing to comment on the scope of the Quantum program, the NSA said its actions were not comparable to China’s.

“NSA’s activities are focused and specifically deployed against — and only against — valid foreign intelligence targets in response to intelligence requirements,” agency spokeswman Vanee Vines, said in a statement. “We do not use foreign intelligence capabilities to steal the trade secrets of foreign companies on behalf of — or give intelligence we collect to — U.S. companies to enhance their international competitiveness or increase their bottom line.”

Over the past two months, parts of the program have been disclosed in documents from the trove leaked by Edward Snowden, the former NSA contractor. The New York Times withheld some of those details, at the request of U.S. intelligence officials, when it reported, in the summer of 2012, on U.S. cyberattacks on Iran.

President Obama is scheduled to announce on Friday what recommendations he is accepting from an advisory panel that proposed changes to NSA operations.

The panel agreed with Silicon Valley executives that some techniques developed by the agency to find flaws in computer systems undermine global confidence in a range of U.S.-made information products.

The NSA’s efforts to reach computers unconnected to a network rely on a century-old technology updated for modern times: radio transmissions.

A catalog produced by the agency that was part of the Snowden documents has many devices.

One looks like a normal USB plug but has a tiny transceiver buried in it. According to the catalog, it transmits information swept from the computer “through a covert channel” that allows “data infiltration and exfiltration.”

Another variant of the technology involves tiny circuit boards that can be inserted in a laptop computer — either in the field or when they are shipped from manufacturers — so that the computer is broadcasting to the NSA even while the computer’s user enjoys the false confidence that being walled off from the Internet constitutes real protection.