The National Security Agency monitored the phone conversations of 35 world leaders after being given the numbers by an official in another US government department, The Guardian newspaper said according to a classified document provided by whistleblower Edward Snowden.

The confidential memo reveals that the NSA encourages senior officials in its "customer" departments, such the White House, State and the Pentagon, to share their "Rolodexes" so the agency can add the phone numbers of leading foreign politicians to their surveillance systems.

The document notes that one unnamed US official handed over 200 numbers, including those of the 35 world leaders, none of whom is named. These were immediately "tasked" for monitoring by the NSA.

"In one recent case," the memo notes, "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."

But the NSA memo, dated October 2006, acknowledges that eavesdropping on the numbers had produced "little reportable intelligence".