German news magazine Der Spiegel says it has seen a secret document showing that United States intelligence bugged European Union offices and gained access to internal computer networks.
The magazine said the claims were based on confidential documents it was shown by the fugitive former security analyst Edward Snowden.
The US National Security Agency document allegedly outlined how it spied on EU internal computer networks, referring to the bloc as a "target".
Der Spiegel says US officials spied on their European counterparts, installing bugs and accessing computer networks in their offices in Washington and monitoring phone calls at an office in Brussels.
Martin Schulz, the president of the European Parliament, told Reuters 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 a statement.
Edward Snowden is believed to be staying at Moscow's airport, having arrived there last weekend from Hong Kong, where he had been staying since he revealed details of top secret US surveillance programmes.
The US has charged him with theft of government property, unauthorised communication of national defence information and wilful communication of classified communications intelligence.
Mr Snowden has requested asylum in Ecuador, which has said it is willing to consider the request but only when he is physically in the Latin American country.
