The White House was scrambling to head off a major diplomatic incident on Wednesday night after the German government said it had received information that US spies had tapped Angela Merkel's personal mobile phone for years.
In the most significant protest by a world leader since Edward Snowden began leaking details of America's global surveillance network, Merkel called President Barack Obama and demanded his personal assurances she was not being monitored.
The president promised the chancellor that US intelligence "is not monitoring and will not monitor" her communications. But the White House did not explicitly rule out the possibility she had been bugged in the past. German diplomatic sources said they were still not satisfied with the White House's explanation and would demand further clarification over the "completely unacceptable" allegations.
The German government said it had received information that the National Security Agency had targeted Merkel's personal mobile over a series of years in an effort to gain diplomatic intelligence.
Steffen Seibert, a spokesman for Merkel, said the spying "would be a serious breach of trust" and that the German leader "unequivocally disapproves of such practices, should they be confirmed, and regards them as completely unacceptable".
He said: "Among close friends and partners, as the Federal Republic of Germany and the US have been for decades, there should be no such monitoring of the communications of a head of government."
The German government did not elaborate on what information it had been given but the call to Obama appeared to have been prompted by information from Der Spiegel, a news magazine that has begun publishing information from Snowden's stolen classified US files.
Der Spiegel said it was the cause of the government's sharp reaction but did not elaborate on what evidence it had. Merkel telephoned the president on Wednesday afternoon, shortly before he went into a sensitive meeting with Nawaz Sharif, the Pakistani prime minister, who was expecting to confront him about US drone strikes.
"Today President Obama and Chancellor Merkel spoke by telephone regarding the allegations that the US National Security Agency intercepted the communications of the German chancellor," said Jay Carney, the White House press secretary.
"I can tell you that the president assured the chancellor that the United States is not monitoring and will not monitor the communications of the chancellor." Carney did not answer a question about whether the NSA may have intercepted Merkel's communications as part of a broader eavesdropping sweep.
He also did not rule out the possibility that the US had in the past monitored her telephone, a nuance that was picked up by German diplomats. "I would just urge you to look at the tenses used by the White House," one German source told The Daily Telegraph.
"They speak only about the present and the future - we 'are not' monitoring and we 'will not' monitor, but not about the past. "The clarification we are seeking is about the issue overall." The row with Germany comes two days after Le Monde, the French newspaper, published claims that the NSA was collecting millions of records of French telephone calls.
"This sort of practice between partners that invades privacy is totally unacceptable and we have to make sure, very quickly, that this no longer happens," said Laurent Fabius, the French foreign minister. Charles Rivkin, the US ambassador to Paris, was summoned to the French foreign ministry to address the claims.
However, Merkel's personal phone call to Mr Obama is the most direct protest by a fellow world leader. Last month, Dilma Rousseff, the Brazilian president, postponed a planned state visit to Washington in protest at claims the US was intensely spying on her country. As revelations of US spying mounted in recent months, Obama ordered a review of how the NSA conducts its intelligence gather.
"The US is reviewing the way that we gather intelligence to ensure that we properly balance the security concerns of our citizens and allies with the privacy concerns that all people share," Carney said.