Spain has summoned the US ambassador to discuss alleged spying on Spanish citizens.
Foreign affairs minister José Manuel García-Margallo y Marfil said that, if it is true, the action was unacceptable behaviour by an ally.
Spanish media reports have claimed that the US National Security Agency secretly monitored 60 million phone calls in Spain during a single month last year.
Spanish newspapers said details of the alleged monitoring were included in documents provided by fugitive US whistleblower Edward Snowden.
The content of the phone calls were not recorded during the alleged monitoring, but the duration of the calls and the serial numbers of the phones involved were.
The revelations come following similar allegations of US surveillance in France and Germany.
Spain's government has so far said it was not aware its citizens had been spied on by the NSA.
It has resisted calls from Germany for the European Union's 28 member states to reach a "no-spy deal", similar to an agreement Germany and France are seeking.
Prime Minister Mariano Rajoy said the country was looking for more information.
An EU parliamentary delegation is due to meet United States security officials and members of congress later to express concern over alleged spying.
El Mundo newspaper has reproduced a graphic, which it said was an NSA document showing the agency had spied on 60.5 million phone calls in Spain between 10 Dec 2012 and 8 Jan this year.
The newspaper said it had reached a deal with Glenn Greenwald, the Brazil-based journalist who has worked with other media on information provided to him by Mr Snowden, to get access to documents affecting Spain.
Spain's European secretary of state and the US ambassador in Spain are scheduled to meet today, after Mr Rajoy said he would seek more details from the ambassador.
"We'll see once we have more information if we decide to join with what France and Germany have done," Mr Rajoy told a news conference in Brussels on Friday.
Meanwhile, The White House said there may need to be additional constraints placed on America's spy agencies after a series of embarrassing disclosures about the broad scope of US intelligence gathering.
President Barack Obama has full confidence in the director of the National Security Agency, General Keith Alexander, and other NSA officials, said White House spokesman Jay Carney.
He added that there should be a balance between the need to gather intelligence and the need for privacy.
"We recognise there needs to be additional constraints on how we gather and use intelligence," Mr Carney said.
A White House review of US surveillance capabilities is well under way and should be completed by the end of the year, Mr Carney said.