The Wednesday report, based on secret NSA documents leaked by former contractor Edward Snowden, appears to show the agency has used weak restrictions on its overseas activities to exploit major US companies' data to a far greater extent than realized.
Previously reported programs included those that allowed easy searches of Google's, Yahoo's and other Internet giants' material based on court orders. But since the interception in the newly disclosed effort, code named MUSCULAR, occurs outside the United States, there is no oversight by the secret intelligence court.
The Post said the operation gained access to a cable or switch that relayed the traffic through an unnamed telecommunications provider.
"We are outraged at the lengths to which the government seems to have gone to intercept data from our private fiber networks, and it underscores the need for urgent reform," said Google chief legal officer David Drummond. Google said it had not been aware of the program, although it recently began speeding its efforts to encrypt internal traffic.
Like other companies, Google and Yahoo constantly send data over leased and shared or exclusive international fiber-optic telecommunication lines as they synchronize information.
The newly disclosed program, operated jointly with the United Kingdom's Government Communications Headquarters, or GCHQ, amassed 181 million records in one recent 30-day span, according to one document reported by the Post. It could not be learned how much of that included material from US residents, how the agency redacted data on them or how much of the information was retained.
Yahoo in January will begin encrypting users' email as it moves to the company, but it declined to say whether it would go further and keep email encrypted as it moves within Yahoo.
The report is likely to add to growing tensions between the US intelligence establishment and the tech companies, which have been struggling to assure customers overseas that they need not fear US spying.
Senate Judiciary Committee Chairman Patrick Leahy said he would ask for an administration briefing on the program because millions of US residents could have had their communications monitored daily.
NTJ/NJF