An undated aerial handout photo shows the National Security Agency (NSA) headquarters building in Fort Meade, Maryland - Reuters

The European Union is demanding "full clarification" from the US over the latest claims that the NSA extensively spied on EU authorities, a leak that could prove damaging to US-EU relations.

The German magazine Der Spiegel reported that the leaked NSA documents show the agency has been involved in bugging the EU representation in Washington and infiltrating computer networks. The magazine says it obtained the documents from whistleblower Edward Snowden.

If the disclosures turn out to be true, they will have a "severe impact", said the head of the European Parliament, Martin Schulz.

"On behalf of the European Parliament, I demand full clarification and require further information speedily from the US authorities with regard to these allegations," said Schulz in a statement.

Washington, which has been outraged by the leaks, has so far not made any comment on the latest claims.

"The document suggests that in addition to installing bugs in the building in downtown Washington, DC, the EU representation's computer network was also infiltrated. In this way, the Americans were able to access discussions in EU rooms as well as emails and internal documents on computers," says the Der Spiegel report, adding that the magazine has seen the documents in "parts".

"If these reports are true, it's disgusting. The United States would be better off monitoring its secret services rather than its allies," Luxembourg Foreign Minister Jean Asselborn has been quoted as saying in the report.