EU Justice Commissioner Viviane Reding on Sunday warned that landmark negotiations with the United States to create a vast free trade zone could be affected if media reports that Washington had bugged EU premises proved true.
"There should be no spying between partners. We can't negotiate a large transatlantic market if there is any doubt that our partners are bugging the offices of European negotiators," Reding said at a meeting with EU citizens in Luxembourg, according to her spokesperson.
European Union countries have responded with dismay to a report in German weekly Der Spiegel this weekend which detailed covert surveillance by the US National Security Agency (NSA) on EU diplomatic missions in the United States and in Brussels.
Der Spiegel said its claims are based on confidential NSA documents, some of which it said it had been able to consult via fugitive leaker Edward Snowden.
One document, dated September 2010 and classed as "strictly confidential", describes how the NSA kept tabs on the EU's mission in Washington, Der Spiegel said.
Microphones were installed in the building and the computer network was infiltrated, giving the agency access to emails and internal documents.
The EU delegation at the United Nations was subject to similar surveillance, Der Spiegel said, adding that the spying also extended to the 27-member bloc's Brussels headquarters.
The allegations have put a strain on EU-US ties less than two weeks after they launched long-awaited formal negotiations to create the world's biggest free trade zone.
Trade in goods between the United States and the 27-country EU last year was worth some 500 billion euros ($670 billion), with another 280 billion euros in services and trillions in investment flows.
The EU says establishing a Free Trade Agreement would add about 119 billion euros annually to the EU economy, and 95 billion euros for the United States.
The first round of talks aimed at creating jobs and boosting the fragile global economy is expected to take place in Washington next month.