Top European Union officials have reacted angrily to revelations that the United States has wiretapped its buildings in Brussels and Washington, saying the allegations could have a “severe impact” on transatlantic trade negotiations, which were launched amid much fanfare less than two weeks ago.

The United States has bugged European Union offices and gained access to EU internal computer networks, according to secret documents cited in the German weekly Der Spiegel on Saturday, the latest in a series of exposures of alleged US spy programmes.

The Justus Lipsius building in Brussels and the EU delegation in Washington were among the “targets” of US wiretapping, according to the documents obtained by the German magazine.

Der Spiegel quoted from a September 2010 "top secret" US National Security Agency (NSA) document that it said fugitive former NSA contractor Edward Snowden had taken with him, and the weekly's journalists had seen in part.

The document outlines how the NSA bugged offices and spied on EU internal computer networks in Washington and at the United Nations, not only listening to conversations and phone calls but also gaining access to documents and emails.

The document explicitly called the EU a "target".

And in the latest series of revelations, Der Spiegel said the US on average tapped half a billion phone calls, e-mails and text messages in Germany in a typical month.

Schulz: ‘This should stop immediately’

A spokesman for the Office of the US Director of National Intelligence had no comment on the Der Spiegel story.

Martin Schulz, the president of the European Parliament, said he was “deeply worried and shocked” by the allegations, adding 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.

Luxembourg Foreign Minister Jean Asselborn told Der Spiegel: "If these reports are true, it's disgusting.

"The United States would be better off monitoring its secret services rather than its allies. We must get a guarantee from the very highest level now that this stops immediately."

Snowden's disclosures in foreign media about US surveillance programmes have ignited a political furore in the United States and abroad over the balance between privacy rights and national security.

EU buildings targeted from NATO compound in Brussels

According to Der Spiegel, the NSA also targeted telecommunications at the Justus Lipsius building in Brussels, home to the European Council, the collective of EU national governments.

Without citing sources, the magazine reported that more than five years ago security officers at the EU had noticed several missed calls and traced them to NSA offices within the NATO compound in Brussels.

Each EU member state has rooms in Justus Lipsius with phone and internet connections, which ministers can use.

Germany, France branded as ‘third class’ partners

Exposing the latest details in a string of reputed spying programmes, Der Spiegel also said the United States tapped half a billion phone calls, e-mails and text messages in Germany in a typical month and has classed its biggest European ally as a target similar to China.

The document Spiegel cited showed that the United States categorised Germany as a "third-class" partner and that surveillance there was stronger than in any other EU country, similar in extent to China, Iraq or Saudi-Arabia.

"We can attack the signals of most foreign third-class partners, and we do it too," Der Spiegel quoted a passage in the NSA document as saying.

It said the document showed that the NSA monitored phone calls, text messages, e-mails and internet chat contributions and has saved the metadata – that is, the connections, not the content – at its headquarters.

On an average day, the NSA monitored about 20 million German phone connections and 10 million internet data sets, rising to 60 million phone connections on busy days, the report said.

While it had been known from disclosures by Snowden that the United States tapped data in Germany, the extent was previously unclear.

News of the US cyber-espionage programme Prism and the British equivalent Tempora have outraged Germans, who are highly sensitive to 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.

In France, Der Spiegel reported, the United States taps about 2 million connection data a day. Only Canada, Australia, Britain and New Zealand – whose intelligence agencies have close contacts with their US counterparts – were explicitly exempted from spy attacks.