Germany's Foreign Ministry has summoned the US ambassador following allegations that American intelligence may have tapped Chancellor Angela Merkel's mobile phone.
A senior lawmaker on Thursday expressed concern at the White House's statement that it is not monitoring and would not monitor Merkel's communications, a response that did not address what might have happened in the past.
The defence minister added that Europe cannot simply return to business as usual in trans-Atlantic ties following a string of reports that the US was spying on its allies.
Merkel's government said she complained to President Barack Obama on Wednesday after receiving information her mobile phone may have been monitored.
It would not elaborate, but German news magazine Der Spiegel, which has published material from National Security Agency (NSA) leaker Edward Snowden, said its research triggered the response.
The Foreign Ministry said US Ambassador John B Emerson was expected to meet on Thursday afternoon Foreign Minister Guido Westerwelle, who would "spell out the position of the German government".
Merkel called Obama to demand an immediate clarification, her spokesman said in a statement.
Between close friends and partners, as Germany and the US have been for decades, there should not be such monitoring of the communications of a government leader.
"She made clear that she views such practices, if proven true, as completely unacceptable and condemns them unequivocally," the statement read.
"Between close friends and partners, as Germany and the US have been for decades, there should not be such monitoring of the communications of a government leader. This would be a grave breach of trust. Such practices should be immediately stopped."
As the NSA scandal raged in Germany during the campaign for the September election, Obama indicated in July that his personal ties with Merkel meant he had no need to spy on her.
"If I want to know what Chancellor Merkel is thinking, I will call Chancellor Merkel," he said then.
The latest developments came as Secretary of State John Kerry, on a visit to Rome, faced fresh questions about mass spying on European allies, based on revelations from Snowden, the fugitive ex-US intelligence operative granted asylum in Russia.
French President Francois Hollande is pressing for the US spying issue to be put on the agenda of a summit of European leaders starting on Thursday.
The two-day Brussels summit, called to tackle a range of social and economic issues, will now be overshadowed by debate on how to respond to the alleged espionage by Washington against two of its closest European Union allies.
For Germany the issue is particularly sensitive. Not only does the government say it has evidence the chancellor's
personal phone was monitored, but the very idea of bugging dredges up memories of eavesdropping by the Stasi secret police in the former East Germany, where Merkel grew up.
French newspaper Le Monde reported earlier this week that the NSA had collected tens of thousands of French phone records.
Just four months ago, Obama defended US anti-terrorism tactics on a visit to Berlin, telling Germans at a news conference with Merkel that Washington was not spying on ordinary citizens.