The European Union has demanded that the United States explain a report in a German magazine that the US is spying on the EU.
The bloc used strong language to confront its closest trading partner over its alleged surveillance activities.
EU High Representative Catherine Ashton said US authorities were immediately contacted about a report in Der Spiegel that the US spy agency had tapped EU offices in Washington, Brussels and at the United Nations.
"As soon as we saw these reports, the European External Action Service made contact with the US authorities in both Washington DC and Brussels to seek urgent clarification of the veracity of and facts surrounding these allegations," Ms Ashton said.
"The US authorities have told us they are checking on the accuracy of the information released yesterday and will come back to us as soon as possible," she said.
France also asked for an explanation. "These acts, if confirmed, would be completely unacceptable," Foreign Minister Laurent Fabius said.
The US government said it would respond through diplomatic channels.
"We will also discuss these issues bilaterally with EU member states," a spokesperson for the Director of National Intelligence said.
"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."
Der Spiegel reported yesterday that the National Security Agency bugged EU offices and gained access to EU internal computer networks, the latest revelation of alleged US spying that has prompted outrage from EU politicians.
The magazine followed up today with a report that the US agency taps half a billion phone calls, emails and text messages in Germany in a typical month, much more than any other European peer and similar to the data tapped in China or Iraq.
It also uses data from internet hubs in south and west Germany that organise data traffic to Syria and Mali.
Revelations about the US surveillance programme, which was made public 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 the US’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.
Germany's federal prosecutor's office, which has authority in matters of national security, said it was looking into whether or not it should start an investigation.
Criminal charges are expected to be filed, spokeswoman Frauke Koehler said.
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 Adolf Hitler's Nazi regime.
German Chancellor Angela Merkel has not commented on the latest report.
Before a visit by US President Barack Obama earlier this month, Ms Merkel defended governments' monitoring of internet communications, however, and said that the US cyber-snooping had helped prevent attacks on German soil.
She stressed during Mr Obama's visit that there were limits to monitoring but stopped short of pressing the issue hard.
Martin Schulz, president of the EU Parliament and also a German, said if the report was correct, it would have a "severe impact" on relations between the EU and the United States.
He told French radio the United States had crossed a line.
"I was always sure that dictatorships, some authoritarian systems, tried to listen ... but that measures like that are now practiced by an ally, by a friend, that is shocking, in the case that it is true," Mr Schulz said in an interview with France 2.
Some EU policymakers said talks for a free trade agreement between the US and the EU should be put on hold 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.
"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," Ms 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?"