The president of the European parliament has called for full clarification from the US over claims it bugged EU offices in America and accessed computer networks.

Martin Schulz said there would be a severe impact on relations between the European trade bloc and the US if revelations by German magazine Der Spiegel proved to be true.

Der Spiegel reported that the US had bugged offices and gained access to EU internal computer networks, according to secret documents, the latest in a series of exposures of alleged US spying.

The magazine quoted from a "top secret" US National Security Agency (NSA) document from September 2010 that it said former NSA contractor Edward Snowden had taken with him, and which its journalists had seen in part.

Der Spiegel said the document outlines how the NSA bugged offices and spied on EU internal computer networks in Washington and at the UN, listening to conversations and phone calls and gaining access to documents and emails. It said the document explicitly called the EU a target.

A spokesman for the office of the US director of National Intelligence did not comment on the Der Spiegel article.

Schulz 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.

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

Luxembourg's 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 the Guardian about US surveillance programmes have ignited a political furore in the US and abroad over the balance between privacy rights and national security.

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.

Snowden, a US citizen, went to Hong Kong in May, weeks before the publication in the Guardian of details he provided about secret government surveillance of internet and phone traffic. He has been in a Moscow airport transit area since last weekend. The government of Ecuador is reviewing his request for asylum.