Senior German officials will travel to the US "shortly" to talk to the White House and the National Security Agency about spying allegations, including how chancellor Angela Merkel's mobile phone was allegedly monitored by the NSA.
German government spokesman Georg Streiter said the heads of its foreign and domestic intelligence agencies would participate in the talks. He did not give a specific date for the trip, saying it was being arranged on "relatively short notice".
European Union leaders have vowed to maintain a strong transatlantic partnership despite their anger over allegations of widespread US spying on allies, but France and Germany say new surveillance rules should be agreed with the US this year.
Mrs Merkel's unusually stern remarks as she arrived at the European Union gathering in Brussels indicated she was not placated by a phone conversation she had on Wednesday with president Barack Obama, or his personal assurances that the US was not listening in on her calls now.
"We need trust among allies and partners," she told reporters. "Such trust now has to be built anew. This is what we have to think about."
"The United States of America and Europe face common challenges. We are allies. But such an alliance can only be built on trust. That's why I repeat again: spying among friends, that cannot be."
The White House may soon face the wrath of other heads of state and government after The Guardian said it obtained a confidential memo suggesting the NSA was able to monitor 35 world leaders' communications in 2006.
The memo said the NSA encouraged senior officials at the White House, Pentagon and other agencies to share their contacts so the spy agency could add foreign leaders' phone numbers to its surveillance systems, the report said.
The Guardian did not identify who was reportedly bugged, but said the memo termed the pay-off very meagre. "Little reportable intelligence" was obtained, it said.
Other European leaders arriving for the 28-nation meeting echoed Mrs Merkel's displeasure. Swedish prime minister Fredrik Reinfeldt called it "completely unacceptable" for a country to eavesdrop on an allied leader.
If reports that Mrs Merkel's phone had been tapped were true, "it is exceptionally serious," Dutch premier Mark Rutte said.
"We want the truth," Italian prime minister Enrico Letta said. "It is not in the least bit conceivable that activity of this type could be acceptable."
Austria's foreign minister Michael Spindelegger said: "We need to re-establish with the US a relationship of trust, which has certainly suffered from this."
France, which also vocally objected to allies spying on each other, asked that the issue of reinforcing Europeans' privacy in the digital age be added to the agenda of the two-day summit.
Before official proceedings got under way, Mrs Merkel held a brief one-to-one meeting with French president Francois Hollande and discussed the spying controversy.
After summit talks that lasted until early today, Herman Van Rompuy, the European Council president, announced at a news conference that France and Germany were seeking bilateral talks with the United States to resolve the dispute over electronic spying by "secret services" by the end of this year.
"What is at stake is preserving our relations with the United States," Mr Hollande said at his own early-morning news conference. "They should not be changed because of what has happened. But trust has to be restored and reinforced."
"It's become clear that for the future, something must change - and significantly," Mrs Merkel said. "We will put all efforts into forging a joint understanding by the end of the year for the co-operation of the (intelligence) agencies between Germany and the US and France and the US, to create a framework for the co-operation."
The leaders' statements and actions indicated that they had not been satisfied with assurances from Washington. On Wednesday, White House spokesman Jay Carney said Mr Obama personally assured Mrs Merkel that her phone was not being listened to now and would not be in the future.
Asked whether the Americans had monitored Mrs Merkel's previous communications, Mr Carney did not rule it out. "We are not going to comment publicly on every specified alleged intelligence activity," he said.
But while the White House was staying publicly silent, Mr Carney said the Obama administration was discussing Germany's concerns "through diplomatic channels at the highest level", as it was with other US allies worried about the alleged spying.
Mrs Merkel said: "We are seeking a basis for co-operation between our (intelligence) services, which we all need and from which we have all received a great deal of information ... that is transparent, that is clear and is in keeping with the character of being partners."