U.S. Ambassador to Spain James Costos made a commitment Monday to Spanish Secretary of State for the European Union Iñigo Mendez de Vigo to clear up any "doubts" Madrid might have about alleged spying by the United States in Spain, the Foreign Ministry said in a statement.

Spanish officials met with the ambassador and made it clear that it was important to preserve trust in bilateral relations and to learn the extent of practices that, if proven to be true, would be "improper and unacceptable between partners and friendly countries," the ministry said.

Costos, who met with Mendez de Vigo at the Foreign Ministry for about 40 minutes, said he would inform his government of the Spanish government's concerns about the alleged spying.

Spanish Foreign Minister Jose Manuel Garcia-Margallo, who is on an official visit to Poland, said Monday that Spain expressed its "serious concerns" about the spying allegations to Costos.

The allegations, if confirmed, could lead to "the rupture of the climate of trust" between Madrid and Washington, Garcia-Margallo said.

Spain, however, still does "not have official confirmation" of the alleged spying, Garcia-Margallo said in a joint press conference with Polish Foreign Minister Radoslaw Sikorski.

Reports in the Spanish press said the U.S. National Security Agency, or NSA, monitored millions of telephone calls in Spain.

The NSA intercepted more than 60 million telephone calls in Spain between December 2012 and January 2013, the El Mundo newspaper reported on Monday, citing some of the documents leaked by former U.S. intelligence operative Edward Snowden.

An NSA document, titled "Spain - last 30 days," summarizes the flow of calls, but not their content, as well as the telephone numbers monitored, their location and the duration of the calls, blogger Glenn Greenwald said.

The NSA also monitored people's activities on the Internet, e-mails and social-networking sites, such as Facebook and Twitter. EFE