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 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 on Wednesday accused the US of tapping her mobile phone.
After Merkel's allegations became public, White House press secretary Jay Carney issued a statement that said the US "is not monitoring and will not monitor" the German chancellor's communications. But that failed to quell the row, as officials in Berlin quickly pointed out that the US did not deny monitoring the phone in the past.