French President Francois Hollande has said allegations that the US bugged European embassies could threaten a huge planned trade deal.
Negotiations over the EU-US pact, the biggest bilateral deal ever negotiated, are due to start on 8 July.
Mr Hollande said there could be no negotiations without guarantees spying would stop "immediately".
US Secretary of State John Kerry said earlier that activities to protect national security were "not unusual".
German Chancellor Angela Merkel's spokesman, Steffen Seibert, said "bugging friends is unacceptable… we are no longer in the Cold War".
He added that Germany wanted the deal to go ahead but "mutual trust is necessary in order to come to an agreement".
Italy's Foreign Minister Emma Bonino said Rome had requested from Washington "clarification of a very thorny affair".