The US has been accused of tapping the phones of 35 world leaders following significant developments in the long-running PRISM scandal.
After accusations earlier in the week by the German government the NSA had monitored German chancellor Angela Merkel's mobile, it has now emerged the policy of tapping the phones of US allies was commonplace.
The allegations, published in the Guardian, seem to show US officials were encouraged to hand over their contact lists to fuel the NSA's espionage efforts. In one example, an unnamed official passed on more than 200 phone numbers including those of 35 as yet unidentified world leaders.
The Guardian's source was a 2006 memo issued to staff in the NSA's Signals' Intelligence Directorate, which said while most of the 200 numbers handed over were publicly available, 43 were "previously unknown" and would be "tasked". Despite their efforts, the memo admits phone tapping offered "little reportable intelligence".
White House press secretary Jay Carney told reporters: "The revelations have clearly caused tension in our relationships with some countries, and we are dealing with that through diplomatic channels," the Guardian said.
"These are very important relations both economically and for our security, and we will work to maintain the closest possible ties."
Both the French and German governments have urged the White House to make amends to ensure diplomatic relationships are not further damaged. Chancellor Merkel said: "Spying among friends: that cannot be. It's become clear that, for the future, something must change – and significantly."
French president François Hollande added: "What is at stake is preserving our relations with the United States. They should not be changed because of what has happened. But trust has to be restored and reinforced."
US spy agencies had already damaged its relations with the European Union, with reports earlier this year that it had monitored EU offices and tapped staff members' telephones.