German Chancellor Angela Merkel says she has made it clear to U.S. President Barack Obama that spying on allies is unacceptable.
Speaking Thursday as she arrived at a summit of the European Union's 28 leaders in Brussels, Ms. Merkel said she told Mr. Obama during a telephone call Wednesday that "spying on friends is not acceptable at all."
"We need to have trust in our allies and partners and this trust must now be established once again," she said.
The two leaders spoke Wednesday after allegations emerged that the U.S. National Security Agency had monitored Chancellor Merkel's cell phone calls.
During Wednesday's telephone call, Mr. Obama told Ms. Merkel the United States is not monitoring, and will not monitor her communications.
A White House statement said both leaders agreed to intensify their intelligence cooperation to protect the security of both countries and their allies, and the privacy of their citizens.
Early Thursday, Germany's foreign minister summoned the U.S. ambassador to discuss the matter. Foreign Minister Guido Westerwelle was expected to meet with U.S. envoy John Emerson later in the day.
The Obama administration has been denying news reports about many U.S. intelligence activities, as it faces a firestorm of criticism over new revelations that it has spied on its allies. Those reports stem from secret documents leaked by NSA contractor Edward Snowden, who is living in Russia.
President Obama has ordered a review of the way U.S. intelligence is gathered, in an effort White House officials say is intended to ensure a proper balance of security concerns and privacy concerns.
Ms. Merkel had raised concerns about the electronic eavesdropping issue when Mr. Obama visited Germany in June, and has demanded answers from the U.S. government and backed calls for greater European data protection.
French President Francois Hollande has been pressing for the U.S. spying issue to be put on the agenda of the long-planned European leaders summit that starts Thursday.
U.S. intelligence director James Clapper issued a statement Tuesday saying the intelligence gathered by the United States is the type "gathered by all nations" as part of their efforts to combat terrorism and other threats.