The U.S. National Security Agency has put stealthy software in almost 100,000 computers around the world allowing it to carry out surveillance on those devices, the New York Times reported on Tuesday. The NSA has planted most of the software through getting access to computer networks, but has also used a secret technology that allows it entry even to computers not connected to the Internet, the Times said, citing U.S. officials, computer experts and documents leaked by former NSA contractor Edward Snowden.

The technology had been in use since at least 2008 and relied on a covert channel of radio waves transmitted from tiny circuit boards and USB cards secretly inserted in the computers, the report added.

Targets of the program have included units of the Chinese military, Russian military networks as well as systems used by Mexican police and drug cartels, European Union trade institutions and allies such as Saudi Arabia, India and Pakistan.

"NSA's activities are focused and specifically deployed against - and only against - valid foreign intelligence targets in response to intelligence requirements," the Times quoted an agency spokeswoman as saying.