Spain demanded Monday details of any US eavesdropping, saying new reports of mass telephone surveillance in the country would be "inappropriate and unacceptable" if proven to be true.
Spain delivered the message to US ambassador James Costos, who was summoned as outrage spread over US snooping on telephone and online communications of both ordinary citizens and world leaders.
Foreign ministry officials met the US envoy just hours after daily newspaper El Mundo 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 tracked the origin and destination of the calls and their duration but not the content, said El Mundo, which published a classified graph of 30 days of telephone call tracing.
The article was jointly authored by US blogger Glenn Greenwald, who said he had access to the 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 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 diplomatically to address Spain's concerns.
The classified graph published in El Mundo showed the daily volume of calls traced in the 30 days to January 8, 2013.
At its peak, December 11, the graph showed that NSA tracked more than 3.5 million calls on Spanish soil in one day.
Though not shown on the graph, the newspaper said such systematic trawling of huge volumes of digital information -- metadata -- would include intercepting personal details through Internet web browsers, emails and social networks such as Facebook and Twitter.
El Mundo said it had reached an agreement with Greenwald for exclusive access to Spain-related spying documents leaked by Snowden.
The paper 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.
Prime Minister Mariano Rajoy said after a summit with fellow European Union leaders in Brussels on Friday that he had no evidence that Spain had been spied on.
Nevertheless, he said Madrid was calling in the US ambassador to seek further information.
At the summit, the 28 European Union leaders approved a statement which said they valued the relationship with the United States but it had to be based on trust and confidence, especially in intelligence matters.
France and Germany are to lead efforts to reach a new understanding with Washington by the end of this year.
Rajoy said that for the moment, Spain would not join Berlin and Paris in this effort and reiterated that intelligence issues were the responsibility of national governments, not the EU.
The Wall Street Journal said Monday that the NSA had tapped the phones of some 35 world leaders but that the White House ended surveillance of several leaders, including Merkel, after an internal mid-year review.
NSA spokeswoman Vanee Vines flatly denied reports in Germany that NSA chief General Keith Alexander had briefed President Barack Obama on the operation against Merkel in 2010 but that the president let the spying continue.