A classified document provided by fugitive intelligence leaker Edward Snowden said the National Security Agency worked closely with the "customer" departments of the US government to secure the phone numbers of leading foreign politicians.
One unnamed US official handed over 200 numbers, including those of the world leaders who were immediately "tasked" for surveillance by the NSA, according to the document.
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The latest revelations come amid a furor over allegations that the United States had tapped the mobile phone of German Chancellor Angela Merkel and after revelations the NSA had monitored the Brazilian and Mexican leaders' communications.
The White House has refused to say whether it had eavesdropped on Merkel in the past following an outcry in Germany.
The NSA memo cited by The Guardian indicated that surveillance was not isolated and the agency routinely tracked the phone numbers of world leaders.
The 2006 memo was circulated among staff in the agency's Signals Intelligence Directorate under the heading "Customers Can Help SID Obtain Targetable Phone Numbers," outlining how agents could mine contact information gathered by officials in other branches of government.
"In one recent case," the memo noted, "a US official provided NSA with 200 phone numbers to 35 world leaders.
"Despite the fact that the majority is probably available via open source, the PCs (intelligence production centers) have noted 43 previously unknown phone numbers. These numbers plus several others have been tasked."
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