A "shocked" European Union on Sunday angrily awaited explanations from Washington following allegations of US spying on EU offices, which could have a "severe impact" on relations.

"I am deeply worried and shocked about the allegations of US authorities spying on EU offices", both in Brussels and the United States, Martin Schulz, president of the European Parliament, said in a statement.

"If the allegations prove to be true, it would be an extremely serious matter which will have a severe impact on EU-US relations."

Schulz demanded full and speedy clarification from the US authorities.

The German weekly Der Spiegel on Sunday published a report it said was based on confidential documents, some of which it had been able to consult via the fugitive US intelligence leaker Edward Snowden.

It was former intelligence analyst Snowden who earlier this month revealed details of the so-called PRISM program operated by the US National Security Agency.

He is currently in the transit area at Moscow airport, seeking a country that will accept his request for asylum.

A document dated September 2010 and classified as "strictly confidential" showed how the NSA spied on the EU's diplomatic mission in Washington, it said.

Microphones were installed in the building and the computer network infiltrated, giving the agency access to e-mails and internal documents.

The EU delegation at the United Nations was subject to similar surveillance, Der Spiegel said. The leaked documents referred to the Europeans as "targets".

Brussels spied on

The spying also extended to the 27-member bloc's Brussels headquarters.

Der Spiegel referred to an incident "more than five years ago" when EU security experts discovered telephone and online bugging devices at the Justus Lipsius building.

In 2003, the EU announced it had found phone taps in the building targeting the offices of several countries, including Britain, France and Germany. It was not immediately clear if Der Spiegel was referring to this case.

US spying was "out of control", said Luxembourg Foreign Minister Jean Asselborn.

"The US would do better to monitor its intelligence services instead of its allies," he added.

Even before the latest allegations, the EU Justice Commissioner Viviane Reding wrote to US attorney general Eric Holder earlier this month, calling for answers about its Internet spy program.

"Fundamentally, this is a question of trust," Reding said in a June 14 speech. "Trust of citizens toward their governments and to the governments of partner nations."

AFP-Xinhua