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, 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 as 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. The revelation is set to add to mounting diplomatic tensions between the US and its allies, after the German chancellor Angela Merkel ... accused the US of tapping her mobile phone. The NSA memo obtained by the Guardian suggests that such surveillance was not isolated, as the agency routinely monitors the phone numbers of world leaders - and even asks for the assistance of other US officials to do so. The memo, dated October 2006 and which was issued to staff in the agency's Signals Intelligence Directorate (SID), was titled "Customers Can Help SID Obtain Targetable Phone Numbers". In the wake of the Merkel row, the US is facing growing international criticism that any intelligence benefit from spying on friendly governments is far outweighed by the potential diplomatic damage.