The National Security Agency (NSA) is tracking the location and movement of billions of mobile phones outside the United States in an effort to find suspicious travel patterns or coordinated activities by intelligence targets, according to secret documents leaked by former NSA contractor Edward Snowden.

To carry out the tracking, the agency collects five billion records on mobile phones outside the US each day from taps on fibre-optic cables and other communication conduits that carry mobile-phone traffic, the documents say. The Washington Post, which reported the overseas collection on Wednesday, said an analytic computer program called CO-TRAVELER crunches the data of billions of unsuspecting people, building patterns of relationships between them based on where their phones go.

In October, The New York Times reported that the NSA carried out a secret test project in 2010 and 2011 to collect large amounts of data on the location of Americans’ mobile phones inside the US.

Mr James Clapper Jr, the Director of National Intelligence, confirmed the existence of the test programme, but said it was never put into practice.

The latest revelation is certain to stoke resentment by foreigners because the scope of NSA spying overseas has seemed to widen with the leak of each new document.

“The government should be targeting its surveillance at those suspected of wrongdoing, not assembling massive associational databases that, by their very nature, record the movement of a huge number of innocent people,” said Ms Catherine Crump, Staff Attorney for the American Civil Liberties Union. Agencies