With the White House doing its best to pour cold water on allegations that US intelligence had tapped the mobile phone of German Chancellor Angela Merkel, new claims have emerged of US spying on 35 world leaders.
The phones were monitored after the US National Security Agency was given the personal phone numbers of the leaders by another US official, according to a 2006 memo leaked to The Guardian by former NSA employee Edward Snowden.
Senior officials of US agencies such as the White House, State Department and Pentagon were encouraged to hand over the contact lists to the NSA in order to facilitate the phone tapping, The Guardian reported.
''We are not going to comment publicly on every specific alleged intelligence activity, and as a matter of policy we have made clear that the United States gathers foreign intelligence of the type gathered by all nations,'' a White House spokeswoman said.
Earlier revelations that the NSA had spied on the leaders of Mexico, Brazil and, most recently, Germany have caused Washington significant embarrassment, and prompted Dr Merkel to openly criticise the US.
The revelations have also complicated delicate negotiations with Germany and other European Union nations over free trade and data-sharing agreements.
Speaking on a panel in Washington hours before the revelations, former secretary of state Madeleine Albright said it was normal practice for nations to spy upon one another.
''Let me just say this, this is not a surprise to people. Countries spy on each other,'' she said.
The revelations by Mr Snowden, now a fugitive in Russia, had nevertheless caused serious harm to the US, she said, and made diplomatic efforts far more difficult. ''This is my personal opinion, glorifying Snowden is a mistake. I think that what he has done is a criminal act and it has hurt us very, very badly,'' she said.
She shared the stage at the left-leaning Centre for American Progress with former prime minister Julia Gillard, who made light of the issue, saying if her phone had been tapped while she was leader, all they would have heard was praise for Barack Obama.
Among diplomatic and intelligence commentators the reaction has been mixed. Some have highlighted the damage and embarrassment caused, others have noted that espionage between nations, even friendly ones, is standard practice.
''There's absolutely nothing shocking here at all,'' John Schindler, professor at the US Naval War College and, like Mr Snowden, a former NSA analyst, told Discovery news.
''This is what intelligence services are supposed to be doing. The French do the exact same thing. Everyone does this. The NSA is just better at it than many other countries. It's the flipside of modern telecommunications. All advanced countries have modern [signals intelligence] capabilities. Those are always directed at foreign countries with which they have economic and political interests.''
European leaders appear unconvinced by the notion such activity is standard. On Thursday, Dr Merkel reiterated her objections and the leaders of Sweden, Austria and Italy made statements conveying their outrage.
''We want the truth,'' Italian Prime Minister Enrico Letta said. ''It is not in the least bit conceivable that activity of this type could be acceptable.''
The Guardian story did not say which leaders had their phones tapped, but the documents suggest they were friendly nations.
An earlier report suggested Australia was protected from US espionage as part of a spying alliance known as the ''Five Eyes'', comprising the US, Canada, Britain, Australia and New Zealand.
The White House has yet to respond to a query from Fairfax Media as to whether the present or former Australian prime ministers were targeted.