The US National Security Agency monitored the telephone conversations of 35 world leaders after obtaining their numbers from an official in another government department, according to a leaked document from the former US intelligence operative Edward Snowden.
The Guardian reported that a confidential memo shows the NSA encouraged senior officials in departments such as the White House, the State Department and the Pentagon to provide access to their Rolodexes containing the phone numbers of leading foreign politicians.
One US official alone was said to have passed on 200 numbers, including those of the 35 world leaders, none of whom is named, who were immediately "tasked" for monitoring by the NSA.
The disclosure will heighten the tensions between the US and key European allies, after the German Chancellor Angela Merkel challenged President Barack Obama over claims the Americans tapped her mobile phone.
French President Francois Hollande has earlier called Mr Obama to confront him over allegations that the NSA was targeting the private phone calls and texts of millions of French people.
In a carefully worded statement, the White House said the US "is not monitoring and will not monitor" Mrs Merkel's communications but officials in Berlin pointed out that it did not deny monitoring the phone in the past.
Earlier, it was reported that the US had denied ever spying on Prime Minister David Cameron.
Caitlin Hayden, a spokesman for the National Security Council told the Daily Telegraph: "We do not monitor PM Cameron's communications."