London, Oct 25 (IANS): In a new revelation, whistle-blower and ex-national Security Agency (NSA) contractor Edward Snowden has said the US intelligence agency monitored the phone calls of 35 world leaders after the contacts were provided by an official in a US government department.

The leaked document notes that one unnamed US official handed over 200 phone numbers, including those of the 35 world leaders, none of whom is named, to the NSA, the Guardian reported.

The numbers provided by the government official were immediately "tasked" for monitoring by the NSA.

The confidential memo reveals that the NSA encourages senior officials of the government to share their contacts so that the agency could add the phone numbers of leading foreign politicians to their surveillance systems.

The revelation came just after German chancellor Angela Merkel Wednesday accused the US of tapping her mobile phone, which could potentially further damage diplomatic relations between the US and its allies.

The NSA memo obtained by the newspaper suggests that such surveillance was not isolated, as the agency routinely monitors the phone numbers of world leaders.

The memo, dated October 2006, was issued to staff in the NSA's Signals Intelligence Directorate (SID), and was titled 'Customers Can Help SID Obtain Targetable Phone Numbers'.

The memo gave an example of how data collected from the US officials who mixed with world leaders and politicians could help agency surveillance.

“In one recent case," the memo notes, "a US official provided NSA with 200 phone numbers of 35 world leaders ... Despite the fact that the majority is probably available via open source, the PCs (intelligence production centres) have noted 43 previously unknown phone numbers. These numbers plus several others have been tasked,” the leaked memo stated.