The National Security Agency's surveillance network has the capacity to spy on 75 percent of all U.S. Internet traffic, The Wall Street Journal reports.

Citing current and former NSA officials for the 75 percent figure, the paper reported that the agency can observe more of Americans' online communications than officials have publicly acknowledged.

The NSA's system of programs that filter communications, achieved with the help of telecommunications companies, is designed to look for communications that either start or end abroad, or happen to pass through the U.S. between foreign countries. However, the officials told the Journal that the system's reach is so broad, that it is more likely that purely domestic communications will be intercepted as a byproduct of the hunt for foreign ones.

The NSA defended the program in a statement to Fox News.

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"NSA's signals intelligence mission is centered on defeating foreign adversaries who aim to harm the country. We defend the United States from such threats while fiercely working to protect the privacy rights of U.S. persons. It's not either/or. It's both," the statement said.

The system works by using algorithms that act as filters, designed to let high-value information through amid more benign chatter. However, after the Sept. 11, 2001 terror attacks, a former to intelligence official told the Journal that the government changed its definition of "reasonable" intelligence collection, enabling the NSA to widen the holes in the "filtering" system.

The details are the latest to emerge about the NSA's operations and capabilities, as authorities in the U.S. and other countries try to stop the release of more information about the elaborate surveillance network. Members of Congress on the intelligence committees, as well as past intelligence officials, recently have spoken up in defense of the agency, particularly after a report showing the agency had broken privacy rules and overstepped its authority thousands of times.

The NSA programs described by the Journal differ from the programs described by former NSA contractor Edward Snowden in a series of leaks earlier this summer. Snowden described a program to acquire Americans' phone records, as well as another program, known as PRISM, that made requests from Internet companies for stored data. By contrast, the Internet monitoring systems have the capability to track almost any online activity, so long as it is covered by a broad court order.