The alleged US spying on Spain, if confirmed, would badly damage the current close relations between the two countries, Spanish Foreign Minister Jose Manuel Garcia-Margallo said on Monday.

Garcia-Margallo said that it was an "unacceptable practice" and warned that if it was discovered the United States had spied on Spanish citizens, "it could mean a rupture in the traditional climate of confidence" between the two countries.

Spain would protect itself "at the maximum level" with the country's penal code and could open legal action against the US National Security Agency (NSA), he said.

The Spanish foreign minister made the remarks after a 40-minute meeting with the US Ambassador to Spain James Costos and the Spanish Secretary of State for the European Union Inigo Mendez de Vigo at the offices of the foreign ministry.

Early indications are that the meeting has failed to clear up the doubts over whether the United States has been spying on Spanish politicians, as it is alleged to have done in the case of German Chancellor Angela Merkel, who is thought to have had her phone tapped by US security services for over 10 years.

Garcia-Margallo explained that Costos has not been in his job for long and needed to "ask for the information in order to communicate it to the Spanish government." The foreign minister refused to rule out the possibility of ordering an investigation by Spanish intelligence organization CNI if it was not forthcoming.

The meeting came on the day that former Guardian journalist, Glenn Greenwald, the man who helped former CIA contractor Edward Snowden leak documents on the extent of US spying, published an article in Spain's El Mundo newspaper alleging that the NSA had spied on over 60 million telephone calls in Spain between Dec. 10, 2012 and Jan. 8, 2013.

Greenwald's article refers to a document entitled "Spain last 30 days," which reportedly reflects the calls which were monitored by the NSA. Although it does not give details of the contents of these calls, it does detail the numbers, where the calls were made from and the duration of the conversations.

The espionage also allegedly includes intrusions into personal information through internet navigators which were used to access private e-mail accounts as well as accounts in the social networking sites, Twitter and Facebook.