Senior European Union politicians say they are shocked at reports that United States intelligence agents bugged EU offices on both sides of the Atlantic.
The president of the European Parliament, Martin Schulz, said he was "deeply worried and shocked about the allegations of U.S. authorities spying on EU offices'' made in a report published Sunday by German news weekly Der Spiegel.
Schulz said if the reports were confirmed "it would be an extremely serious matter which will have a severe impact on EU-US relations".
"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.
German Green Party leaders in the European Parliament, Rebecca Harms and Daniel Cohn-Bendit, called for an investigation and called for existing US-EU agreements on the exchange of bank transfer and passenger record information to be cancelled.
Der Spiegel's article quoted a September 2010 "top secret" document that outlined how the National Security Agency 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, which was leaked by whistleblower Edward Snowden, explicitly called the EU a "target".
If these reports are true, it's disgusting.
The NSA also targeted telecommunications at the Justus Lipsius building in Brussels, home to the European Council, the collective of EU national governments, according to the magazine.
German prosecutors on Sunday said they were deciding whether German laws were broken by the spy programme before considering a formal investigation.
Snowden, a former NSA contractor, earlier this month revealed the existence of the so-called PRISM programme operated by the the US security agency.
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."
The office of the US director of National Intelligence said it would respond to the EU via 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," it added.
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.
Snowden fled to Hong Kong in May, a few weeks before the leaks. He has been holed up in a Moscow airport transit area since last weekend.
The leftist government of Ecuador, which is reviewing his request for asylum, said on Saturday that the US has asked them to refuse the request.