The Spanish government demanded details as it called in US Ambassador James Costos to explain the latest allegations in a growing scandal over US snooping on the telephone and online communications of ordinary citizens and world leaders including German Chancellor Angela Merkel.
The news emerged just as a European Parliament delegation began a three-day mission to Washington to probe the impact of the surveillance on EU citizens' "fundamental rights" and to discuss a threat to suspend an EU-US agreement on the transfer of banking data.
A senior Spanish foreign ministry official met with the US ambassador hours after the El Mundo daily published a classified document purportedly showing that the US security services tracked 60.5 million Spanish telephone calls in a single month.
The National Security Agency (NSA) recorded the origin and destination of the calls and their duration but not the content, said El Mundo, which printed a classified graph showing 30 days of call tracing up to January 8 this year.
The graph illustrated the daily volume of calls monitored in the period, peaking at 3.5 million on December 11th.
Though not shown on the graph, the newspaper said such systematic trawling of huge volumes of digital information — or metadata — would include intercepting personal details through Internet web browsers, emails and social networks such as Facebook and Twitter.
The article was jointly authored by US blogger Glenn Greenwald, who said he had access to previously secret documents obtained by former US intelligence contractor Edward Snowden.
The Spanish foreign ministry said it had underscored with the US ambassador its concern over the reported snooping.
"Spain conveyed to the United States the importance of preserving the climate of trust that governs bilateral relations and of knowing the scale of practices that, if true, are inappropriate and unacceptable between countries that are partners and friends," it said in a statement.
Spain's state secretary for the European Union, Inigo Mendez de Vigo, "urged the US authorities to provide all necessary information about the supposed tapping in Spain", it said.
The White House gave its fullest response yet to the scandal, saying in carefully phrased comments from Obama's spokesman Jay Carney that it must better evaluate the risks and rewards of its spying.
"We... need to ensure that our intelligence resources are most effectively supporting our foreign policy and national security objectives, that we are more effectively weighing the risks and rewards of our activities," Carney said.
"We need to ensure that we are collecting information not just because we can, but because we should, because we need it for our security," he said.
However the White House still refused to reveal whether it had bugged Merkel's phone or comment on any other specifics.
"We hope for an open dialogue"
During a visit to Poland, Spanish Foreign Minister Jose Manuel Garcia Margallo, told journalists that if the reported espionage was confirmed "it could mean a break in the climate of trust that has traditionally reigned in relations between the two countries".
The US ambassador said in a separate statement that some of the security programmes played a "critical role" in protecting Americans and were also instrumental in protecting allied interests.
He promised to work to address Spain's concerns.
El Mundo urged Spanish prosecutors to charge the NSA with spying, saying such tracing of telephone calls without the proper judicial authority amounted to a criminal offence.
In Washington, US lawmakers sought to soothe injured European feelings as they held talks with the parliamentary mission from Brussels.
"We hope for an open dialogue today," US House of Representatives intelligence committee chairman Mike Rogers told reporters as he entered the meeting.
"We're going to find some common ground here today."
The Wall Street Journal said Monday that the NSA had tapped the phones of some 35 world leaders including close ally Merkel, who last week branded the snooping as unacceptable between friends.
President Barack Obama learned of the espionage programme only after an internal mid-year review, and the White House then ordered an end to the spying on some leaders, including Merkel, the Wall Street Journal said.
NSA spokeswoman Vanee Vines flatly denied reports in Germany that NSA chief General Keith Alexander had briefed Obama on the operation against Merkel in 2010 but that the president let the spying continue.
German media had reported at the weekend that eavesdropping on Merkel's phone may have started in 2002, when she was Germany's main opposition leader and three years before she became chancellor.
And the daily Bild am Sonntag quoted US intelligence sources as saying that Obama himself had been informed of the phone tap against Merkel by NSA chief General Keith Alexander in 2010 but allowed it to continue.
Alexander "did not discuss with President Obama in 2010 an alleged foreign intelligence operation involving German Chancellor Merkel, nor has he ever discussed alleged operations involving Chancellor Merkel", Vines said Sunday.
"News reports claiming otherwise are not true."
France and Germany are leading efforts to reach a new understanding with Washington following the approval Friday of a statement from the 28-nation EU saying the bloc valued its relationship with the US, but that it had to be based on trust.