The European Union used unusually strong language to confront its closest trading partner over its alleged surveillance activities.
A spokeswoman for the European Commission said on Sunday the EU contacted US authorities in Washington and Brussels about a report in Der Spiegel magazine that the US secret service had tapped EU offices in Washington and Brussels and at the United Nations.
"We have immediately been in contact with the US authorities in Washington DC and in Brussels and have confronted them with the press reports," the spokeswoman said.
"They have told us they are checking on the accuracy of the information released yesterday and will come back to us," she added in a statement.
Der Spiegel reported on its website on Saturday that the National Security Agency had bugged EU offices and gained access to EU internal computer networks in the latest revelation of alleged US spying that has prompted outrage from EU politicians.
The magazine followed up on Sunday with a report that the US secret service taps half a billion phone calls, emails and text messages in Germany in a typical month and has classed its biggest European ally as a target similar to China.
Revelations about the alleged US spying programme, which became public through documents taken by fugitive former NSA contractor Edward Snowden, have raised a furore in the United States and abroad over the balance between privacy rights and national security.
The extent to which Washington's EU allies are being monitored has emerged as an issue of particular concern.
"If the media reports are correct, this brings to memory actions among enemies during the Cold War. It goes beyond any imagination that our friends in the United States view the Europeans as enemies," said German Justice Minister Sabine Leutheusser-Schnarrenberger.
"If it is true that EU representations in Brussels and Washington were indeed tapped by the American secret service, it can hardly be explained with the argument of fighting terrorism," she said in a statement.
Germans are particularly sensitive about government monitoring, having lived through the Stasi secret police in the former communist East Germany and with lingering memories of the Gestapo of Hitler's Nazi regime.
On Saturday, Martin Schulz, president of the EU Parliament and also a German, said that if the report was correct, it would have a "severe impact" on relations between the EU and the United States.
"On behalf of the European Parliament, I demand full clarification and require further information speedily from the US authorities with regard to these allegations," he said in an emailed statement.
Some policymakers said talks for a free trade agreement between Washington and the EU should be put on ice until further clarification from the United States.
"Partners do not spy on each other," the European commissioner for justice and fundamental rights, Viviane Reding, said 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," Reding said in comments passed on to reporters by her spokeswoman.
The European Parliament's foreign affairs committee head Elmar Brok, from Chancellor Angela Merkel's Christian Democrats. echoed those views.
"The spying has taken on dimensions that I would never have thought possible from a democratic state," he told Der Spiegel.
"How should we still negotiate if we must fear that our negotiating position is being listened to beforehand?" – Reuters