Word that U.S. intelligence agencies may have monitored German Chancellor Angela Merkel's cellphone has infuriated Germans. The Post's Michael Birnbaum in Berlin explains how the disclosure could imperil dealings like a looming free-trade pact. (Nicki Demarco/The Fold/The Washington Post)

The Obama administration’s relationship with Europe appeared to have suffered a major setback Thursday over accusations of eavesdropping, including a new report alleging that the National Security Agency had monitored the phone conversations of more than 30 world leaders.

Furious Europeans threatened to delay trade negotiations over a report that U.S. intelligence agencies listened in on German Chancellor Angela Merkel’s cellphone conversations. German officials launched a legal investigation and said the scandal could disrupt counterterrorism collaboration between the United States and the European Union.

The Web site of Britain’s Guardian newspaper, meanwhile, published what it described as a secret memo in which the NSA encouraged U.S. officials to share their “rolodexes” with the agency. The memo said that in one case, an American official had provided the NSA with 200 phone numbers tied to 35 world leaders.

The memo went on to say that the numbers had resulted in “little reportable intelligence,” apparently because they were not used for “sensitive discussions.”

Erosion of trust

The issue of U.S. surveillance quickly rose to the top of the agenda at a long-planned summit of European leaders in Brussels that began Thursday. Many in attendance said they were shocked that a powerful ally they considered a friend might have tapped their personal communications, long deemed a diplomatic no-no.

“Spying among friends, that just does not work,” Merkel told reporters in Brussels. “The United States and Europe face common challenges,’’ she said, adding: “Trust has to be restored.’’

President Obama has assured Merkel that the United States “is not monitoring and will not monitor’’ her communications, the White House said. But for a second day, the assurances left out discussion of whether the German leader had been monitored in the past.

Asked Thursday at his daily news briefing whether the NSA had spied on Merkel’s phone calls, White House spokesman Jay Carney declined to respond directly.

“We are not going to comment publicly on every specified, alleged intelligence activity. And, as a matter of policy, we have made clear that the United States gathers foreign intelligence of the type gathered by all nations,” Carney said. “We have diplomatic relations and channels that we use in order to discuss these issues that have clearly caused some tension in our relationships with other nations around the world, and that is where we were having those discussions.”

German, French criticism

Analysts said Thursday that although some of Europe’s outrage in recent months over NSA spying was more for domestic consumption than because of genuine surprise, the shock in Germany did stem from a sense that a line had been crossed.

“There was a general assumption that certain kinds of people were off-limits,” said Constanze Stelzenmueller, a security and defense expert at the Berlin office of the German Marshall Fund of the United States, a public policy research group. “No one has a problem with spying on the bad guys. But when you start spying on your partner in leadership, who is presumably not a terrorist, that raises a lot of questions about trust.”

In a flurry of activity Thursday, Merkel met one-on-one with French President François Hollande, who also this week condemned alleged American spying in France. The U.S. ambassador in Berlin was summoned to the Foreign Ministry for consultations.

The suspicions about U.S. monitoring of an allied leader’s personal communications threatened to undermine a host of U.S.-E.U. priorities, including some long-standing joint efforts to combat terrorism. German Justice Minister Sabine Leutheusser-Schnarrenberger on Thursday called for a suspension of a European financial-data-sharing deal with the United States that targets terrorism suspects.

Other European officials said efforts to forge a major U.S.-E.U. trade deal also could be damaged.

“Spying on close friends and partners is totally unacceptable,” German Foreign Minister Guido Westerwelle told reporters in Berlin on Thursday after he met with U.S. Ambassador John Emerson. “This undermines trust, and this can harm our friendship.” Westerwelle said the German government would not have gone public with the issue if it did not have grounds to do so.

Merkel had called Obama on Wednesday to demand an explanation for the allegations. On Thursday, State Department spokeswoman Marie Harf said U.S. intelligence operations weren’t “some big dragnet,” scooping up all communications everywhere. “These are intelligence activities, broadly speaking, with a defined purpose.”

Stewart A. Baker, a former NSA general counsel, said it would not be a surprise if the agency were eavesdropping on Merkel’s calls, though he added that he did not know whether the NSA had done so.

The European warnings about the future of the U.S-E.U. trade deal appeared to deliver a blow to a process that could expand trade between the world’s two biggest economic blocs. The Obama administration has described the pact as a priority.

“This is a moment when we should pause and think over how the free-trade pact is being approached,” Martin Schulz, the president of the European Parliament, told reporters. He said U.S. intelligence agencies were “out of control.’’

“Every country in the world has interests that are opposed to U.S. interests in some respects, even though they might define themselves in other contexts as allies,” Baker said. “The European Union is built on that premise — ‘Of course we’re NATO allies, but of course we’re going to do everything we can to stick it to you in trade.’ ”

Privacy concerns

The allegations that Merkel’s cellphone had been monitored were sure to be particularly galling for a leader who is known for being glued to her mobile device.

The latest accusations appear to have been prompted by an investigation by the German newsmagazine Der Spiegel. The magazine did not say where it had obtained information that Merkel’s phone had been monitored, but it has published several stories based on leaks from former NSA contractor Edward Snowden in recent months.

If the allegations are true, Merkel, an East German native, would not be experiencing phone-tapping for the first time. The Stasi, East Germany’s intelligence service, was notorious for its extensive surveillance of all aspects of the lives of East Germans. Those memories and their country’s Nazi past make Germans particularly sensitive to the idea of being watched by governments, and Germany’s privacy laws are among the strongest in the world.

Even before the latest allegations, German companies were discussing efforts to create a more secure national data network, free from U.S. oversight. To do so, they would need to change the basic architecture of Germany’s Internet infrastructure to ensure that data do not pass through servers outside the country.

“It’s a kind of self-defense,” said Peter Schaar, Germany’s federal data protection chief. “We need practical means to protect ourselves.”

Scott Wilson, Anne Gearan and Ellen Nakashima in Washington contributed to this report.