European leaders reacted with fury Sunday to allegations in a German newsmagazine that the United States had conducted a wide-ranging effort to monitor European Union diplomatic offices and computer networks, with some saying that they expected such surveillance from enemies, not their closest economic partner.

It was the latest fallout from National Security Agency information apparently leaked by Edward Snowden, a former NSA contractor whose detailing of classified information on the agency’s programs has shined a rare light on U.S. surveillance efforts that range far wider than previously understood.

Underscoring the depth of European anger over the allegations, top officials from several European countries said that the reports of spying would figure into the future of transatlantic trade talks that began in June. The efforts would create the world’s largest free-trade zone, and European officials said Sunday that they suspected the target of U.S. intelligence interest was economic information, not military.

“Partners do not spy on each other,” said E.U. Commissioner for Justice Viviane Reding at a public event in Luxembourg on Sunday. “We cannot negotiate over a big transatlantic market if there is the slightest doubt that our partners are carrying out spying activities on the offices of our negotiators.”

Other European leaders said they felt blindsided by the allegations.

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“It is shocking that the United States take measures against their most important, their nearest allies, comparable to measures taken in the past by the KGB, by the secret service of the Soviet Union,” European Parliament President Martin Schulz told reporters in Brussels on Sunday.

“This is not the basis to build mutual trust, this is a contribution to build mutual mistrust,” he said, adding that he felt treated like an “enemy.”

Germany’s Der Spiegel newsmagazine reported this weekend that the NSA had placed listening devices in E.U. diplomatic offices in Washington and New York, had breached an E.U. computer network that provided access to internal e-mails and documents, and had accessed phone lines in E.U. headquarters in Brussels in order to monitor top officials’ phone conversations. The magazine said that it had seen portions of 2010 documents from Snowden, although it did not publish them on its Web site nor did it quote from them directly.

Later Sunday, Britain’s Guardian newspaper published additional information, including portions of an internal NSA presentation that appear to detail several methods by which U.S. intelligence agencies monitored diplomats inside the United States. The Dropmire program apparently monitored communications on an encrypted fax machine used by the E.U. delegation in Washington to communicate with counterparts in Europe.

The Guardian also reported that another document lists 38 embassies and missions that U.S. intelligence agencies were monitoring in some way, including the embassies of U.S. allies France, Italy, Japan, India and South Korea, and others including more traditional antagonists and Middle Eastern countries.

E.U. foreign policy chief Catherine Ashton said in a statement Sunday that she had asked for further information from U.S. officials in Washington and Brussels.

A spokesman from the Office of the Director of National Intelligence said Sunday that the U.S. government would respond through diplomatic channels.

“While we are not going to comment publicly on specific alleged intelligence activities, as a matter of policy we have made clear that the United States gathers foreign intelligence of the type gathered by all nations,” the spokesman said in a statement.

Der Spiegel on Sunday separately reported that the NSA monitored 500 million e-mails, phone calls and text messages in Germany every month, more than any European peer. Germany’s Federal Prosecutor’s Office said Sunday that it would open an inquiry to determine whether charges should be filed. The revelations of U.S. spying have special resonance in Germany, where memories of omnipresent Stasi surveillance in East Germany remain fresh. German Chancellor Angela Merkel was one of the first leaders to demand more information from President Obama when revelations of U.S. surveillance were first disclosed earlier in June.

Merkel did not immediately respond this weekend to the new allegations, although she said ahead of a visit by Obama two weeks ago that U.S. intelligence had previously helped foil terrorist attacks in Germany.

Other German officials said Sunday that they were deeply unhappy.

“If the media reports are correct, the procedures resemble those among enemies during the Cold War,” said German Justice Minister Sabine Leutheusser-Schnarrenberger in a statement Sunday.

But not all European officials seemed shocked Sunday by the fresh revelations.

“E.U. trade negotiators have always assumed someone listened,” said Robert Madelin, the British director general of the European Commission for Communications Networks, Content and Technology, on Twitter on Sunday.

The United States has been trying to track down Snowden, who fled from Hawaii to Hong Kong and is said to be in the transit area of Moscow’s Sheremetyevo International Airport. He had apparently been trying to reach Ecuador, but Sunday comments from Ecuadoran President Rafael Correa made it sound as though the country may be cautious about accepting him.

David Fahrenthold in Washington contributed to this report.