Transatlantic relations plunged at the weekend as Berlin, Brussels and Paris all demanded that Washington account promptly and fully for new disclosures on the scale of the US National Security Agency’s spying on its European allies.

As further details emerged of the huge reach of US electronic snooping on Europe, Berlin accused Washington of treating it like the Soviet Union, “like a cold war enemy”.

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The European commission called on the US to clarify allegations that theNSA, operating from Nato headquarters a few miles away in Brussels, had infiltrated secure telephone and computer networks at the venue for EU summits in the Belgian capital. The fresh revelations in the Guardianand allegations in the German publication Der Spiegel triggered outrage in Germany and in the European parliament and threatened to overshadow negotiations on an ambitious transatlantic free-trade pact worth hundreds of billions due to open next week.

The reports of NSA snooping on Europe – and on Germany in particular – went well beyond previous revelations of electronic spying said to be focused on identifying suspected terrorists, extremists and organised criminals.

Der Spiegel reported that it had seen documents and slides from theNSA whistleblower Edward Snowden indicating that US agencies bugged the offices of the EU in Washington and at the UN in New York. They are also accused of directing an operation from Nato headquarters in Brussels to infiltrate the telephone and email networks at the EU’s Justus Lipsius building in the Belgian capital, the venue for EU summits and home of the European council.

Citing documents it said it had “partly seen”, the magazine reported that more than five years ago security officers at the EU had noticed several missed calls apparently targeting the remote maintenance system in the building that were traced to NSA offices within the Nato compound in Brussels.

Less than three months before a German general election, the impact of the fresh disclosures is likely to be strongest in Germany which, it emerged, is by far the biggest target in Europe for the NSA’s Prismprogramme scanning phone and internet traffic and capturing and storing the metadata.

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The documents reviewed by Der Spiegel showed that Germany was treated in the same US spying category as China, Iraq or Saudi Arabia, while the UK, Canada, Australia, and New Zealand were deemed to be allies not subject to remotely the same level of surveillance.

Germany’s justice minister, Sabine Leutheusser-Schnarrenberger, called for an explanation from the US authorities. “If the media reports are true, it is reminiscent of the actions of enemies during the cold war,” she was quoted as saying in the German newspaper Bild. “It is beyond imagination that our friends in the US view Europeans as the enemy.”

France later also asked the US for an explanation. The foreign minister, Laurent Fabius, said: “These acts, if confirmed, would be completely unacceptable.

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“We expect the American authorities to answer the legitimate concerns raised by these press revelations as quickly as possible.”

Washington and Brussels are scheduled to open ambitious free-trade talks next week after years of arduous preparation. Senior officials in Brussels are worried that the talks will be setback by the NSA scandal. “Obviously we will need to see what is the impact on the trade talks,” said a senior official in Brussels.

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A second senior official said the allegations would cause a furore in the European parliament and could then hamper relations with the US.

However, Robert Madelin, one of Britain’s most senior officials in the European commission, tweeted that EU trade negotiators always operated on the assumption that their communications were listened to.

A spokesman for the European commission said: “We have immediately been in contact with the US authorities in Washington and in Brussels and have confronted them with the press reports. They have told us they are checking on the accuracy of the information released yesterday and will come back to us.”

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There were calls from MEPs for Herman Van Rompuy, president of the European council – who has his office in the building allegedly targeted by the US – and José Manuel Barroso, president of the European commission, to urgently appear before the chamber to explain what steps they were taking in response to the growing body of evidence of US and British electronic surveillance of Europe through the Prism and Temporaoperations.

Guy Verhofstadt, the former Belgian prime minister and leader of the liberals in the European parliament, said: “This is absolutely unacceptable and must be stopped immediately. The American data-collection mania has achieved another quality by spying on EU officials and their meetings. Our trust is at stake.”

Luxembourg’s foreign minister, Jean Asselborn, told Der Spiegel: “If these reports are true, it’s disgusting.” Asselborn called for guarantees from the highest level of the US government that the snooping and spying be halted immediately.

Martin Schulz, the head of the European parliament, said: “I am deeply worried and shocked about the allegations of US authorities spying on EU offices. If the allegations prove to be true, it would be an extremely serious matter which will have a severe impact on EU-US relations.

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“On behalf of the European parliament, I demand full clarification and require further information speedily from the US authorities with regard to these allegations.”

There were also calls for John Kerry, the US secretary of state on his way back from the Middle East, to make a detour to Brussels to explain US activities.

“We need to get clarifications and transparency at the highest level,” said Marietje Schaake, a Dutch liberal MEP. “Kerry should come to Brussels on his way back from the Middle East. This is essential for the transatlantic alliance.”

The documents suggesting the clandestine bugging operations were from September 2010, Der Spiegel said.

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Der Spiegel quoted the Snowden documents as revealing that the US taps half a billion phone calls, emails and text messages in Germany a month. “We can attack the signals of most foreign third-class partners, and we do,” Der Spiegel quoted a passage in the NSA document as saying.

It quoted the document from 2010 as stating that “the European Union is an attack target”.

On an average day, the NSA monitored about 15m German phone connections and 10m internet datasets, rising to 60m phone connections on busy days, the report said.

Officials in Brussels said this reflected Germany’s weight in the EU and probably also entailed elements of industrial and trade espionage. “The Americans are more interested in what governments think than the European commission. And they make take the view that Germany determines European policy,” said one of the senior officials.

Jan Philipp Albrecht, a German Green party MEP and a specialist in data protection, told the Guardian the revelations were outrageous. “It’s not about political answers now, but rule of law, fundamental constitutional principles and rights of European citizens,” he said.

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“We now need a debate on surveillance measures as a whole looking at underlying technical agreements. I think what we can do as European politicians now is to protect the rights of citizens and their rights to control their own personal data.”

Germany has some of the toughest data privacy laws in Europe, with the issue highly sensitive not least because of the comprehensive surveillance by the Stasi in former communist east Germany as well as the wartime experience with the Gestapo under the Nazis.

Der Spiegel noted that so far in the NSA debacle, the chancellor, Angela Merkel, had asked only “polite” questions of the Americans but that the new disclosures on the sweeping scale of the surveillance of Germany could complicate her bid for a third term in September.