German Chancellor Angela Merkel says trust between the US and its partners has to be restored following allegations that American intelligence targeted her mobile phone, and insisted that there must be no "spying among friends".
Mrs Merkel complained to President Barack Obama in a phone call on Wednesday after receiving information her mobile phone may have been monitored. The White House said the US is not monitoring and won't monitor her communications - but didn't address what might have happened in the past.
In her first public comments since news of the allegations emerged, Mrs Merkel said she told Mr Obama that "spying among friends cannot be".
"We need trust among allies and partners," Mrs Merkel said as she arrived at a long-planned summit of the European Union's 28 leaders. "Such trust now has to be built anew. This is what we have to think about."
She stressed that the US and Europe "face common challenges; we are allies." But, she added. "such an alliance can only be built on trust".
In Berlin, the Foreign Ministry summoned the US ambassador to complain, while Germany's defence minister said that Europe can't simply return to business as usual in trans-Atlantic ties following a string of reports that the US was spying on its allies.
Mrs Merkel's chief of staff, Ronald Pofalla, said officials would make "unmistakably clear" to US Ambassador John B Emerson "that we expect all open questions to be answered".
The US Embassy said it had no comment.
Defence Minister Thomas de Maiziere said the alleged surveillance would be "really bad" if confirmed. "The Americans are and remain our best friends, but this is absolutely not right," he said.
"I have reckoned for years with my cellphone being monitored, but I wasn't reckoning with the Americans," said Mr de Maiziere, who was previously Mrs Merkel's chief of staff and Germany's interior minister.
"We can't simply return to business as usual," he said when asked about possible effects on US-German and US-European relations.
This week, France demanded an explanation of a report the US swept up millions of French phone records, and also summoned the American ambassador.
Germany, which has Europe's biggest economy, has been one of Washington's closest allies in Europe. The United States was West Germany's protector during the Cold War and the country is still home to thousands of US troops.
A German parliamentary committee that oversees the country's intelligence service held a meeting on Thursday to discuss the matter, which Mr Pofalla attended.
He said that the government received information from news magazine Der Spiegel on the matter and then launched "extensive examinations" of the material. Der Spiegel has published material from National Security Agency leaker Edward Snowden, but didn't detail its sources on the mobile phone story.
Recalling previous reports to the panel that US authorities have said they didn't violate German interests, committee head Thomas Oppermann said that "we were apparently deceived by the American side." Mr Pofalla said he had ordered a review of previous statements received from the NSA.