By Andrea Mitchell and Erin McClam, NBC News

(NBC) - The United States is scrambling to soothe some of its closest allies, angered as one report after another details vast American spying — including gathering data on tens of millions of phone calls in Spain a single month.

The latest report, published Monday in the Spanish newspaper El Mundo, said that the National Security Agency had collected information on 60 million calls in that country last December.

It followed reports in the last week that the United States spied on leaders of at least 35 countries, and even bugged the personal cellphone of German Chancellor Angela Merkel.

German intelligence chiefs are preparing to visit Washington this week to demand answers. President Barack Obama has had to apologize to Merkel and to the presidents of France and Brazil. The Brazilian president was so angry she canceled a state visit.

The Obama administration and its defenders say that most of the spying is legitimate, for the protection of the United States and its allies.

In a statement Sunday night, U.S. National Security Council spokeswoman Caitlin Hayden said that a White House review is examining "the way that we gather intelligence to ensure that we properly account for the security concerns of our citizens and allies and the privacy concerns that all people share, and to ensure that our intelligence resources most effectively support our foreign policy and national security objectives."

The reports have come from documents leaked by Edward Snowden, the fugitive former NSA contractor. The only clear denial from the NSA has concerned a British report that said that Obama was told three years ago that the agency was eavesdropping on Merkel.

The German newspaper Der Spiegel reported that Obama told Merkel that he would have halted the hacking if he had known about it.

The consequences may be economic, not just diplomatic. The European Union, the United States' largest trading partner, is threatening to cancel trade talks over the revelation.

In the meantime, the U.S. is left to deal with furious allies without knowing what Snowden will reveal next.

"When we're doing this on Germany, on France, on Great Britain and other nations that we've been allied with in fighting Al Qaeda, in invading Libya together, these kinds of things just trample trust," said Steve Clemons, who writes frequently on foreign policy.
Robert Gibbs, a former press secretary for the Obama administration, told TODAY on Monday that "clearly, damage has been done."

"I think we have to evaluate whether the costs of the methods of gathering some intelligence greatly exceeds the benefit of that intelligence, particularly when we're listening in to, apparently, some of our very closest allies," he said.