US 'breach of trust' over spy claims: EU sources
"If it's true that the Americans have been spying on their allies, then there will be a political fallout," a European Union source said, speaking on condition of anonymity.
"This goes far beyond the requirements of national security. It is a breach of trust and we are at the beginning of something very serious," the source said, reacting to reports that Washington had bugged EU offices.
Another source said EU foreign policy chief Catherine Ashton had asked US Secretary of State John Kerry to "clarify as quickly as possible", although Kerry said that seeking information on other countries was "not unusual".
The EU's Justice Commissioner Viviane Reding has taken a harder line, warning that talks to create what would be the world's biggest free trade area, formally launched last month, could be jeopardised if the allegations proved true.
An EU-US working group has been put in place and is due to meet in July to exchange information on the extent of the Prism spying programme and how it affects EU citizens.
Political groups in the European Parliament are also angry and some of them, like the Greens, are calling for a block on US access to European bank data through the Swift system.
The EU so far is officially playing the diplomatic card but it is also pursuing a more hard-nosed approach that for the moment is "complementary", the EU sources said.