The German Foreign Ministry says it has summoned the U.S. ambassador in the wake of allegations that American intelligence may have targeted Chancellor Angela Merkel's cellphone.
The ministry said Ambassador John B. Emerson is expected to meet Thursday afternoon with Foreign Minister Guido Westerwelle, who will "spell out the position of the German government."
The U.S. Embassy said it had no comment.
The move comes after Germany's defense minister said Europe can't simply return to business as usual in its relations with Washington following allegations that U.S. intelligence may have targeted the chancellor's cell phone.
Thomas de Maiziere said that would be "really bad" if confirmed but stressed Thursday that the Americans "are and remain our best friends" and relations will remain stable.
Merkel's government says she complained to U.S. President Barack Obama Wednesday after receiving information her cell phone may have been monitored, and "made clear that she unequivocally condemns and regards such practices as fully unacceptable, if the information is proven true," German government spokesman Steffen Seibert said.
Obama assured Merkel that U.S. agencies were not monitoring her mobile telephone communications, the White House said.
"The president assured the chancellor that the United States is not monitoring and will not monitor the communications of the chancellor," spokesman Jay Carney told reporters.
The White House later declined to clarify to DPA whether Merkel's telephone calls had been monitored in the past.
Asked about possible effects on U.S.-German and U.S.-European relations, de Maiziere told ARD television: "We can't simply return to business as usual. There are allegations in France too."
The White House stressed in a statement that Washington "greatly values our close cooperation with Germany on a broad range of shared security challenges" and that the countries would continue to cooperate on intelligence matters.
The phone call was made a day before Merkel was expected in Brussels for a European Union summit, where EU leaders were expected to discuss data protection in the wake of allegations of widespread US monitoring of internet traffic and phone calls.
German news website Spiegel Online reported that its own inquiry into the alleged monitoring of Merkel's mobile phone prompted the German government reaction.
German security agencies gave no immediate reaction late Wednesday.
The German government said in a statement that as a close ally of the United States, Berlin expects clear rules about activities of intelligence agencies and their future cooperation.
This week, France demanded explanations of a report the U.S. swept up millions of French phone records.