German Chancellor Angela Merkel has called US President Barack Obama after receiving information that the US may have spied on her mobile phone.
A spokesman for Mrs Merkel said the German leader “views such practices… as completely unacceptable”.
Mrs Merkel has called on US officials to clarify the extent of their surveillance in Germany.
The White House said President Obama had told Chancellor Merkel the US was not snooping on her communications.
“The United States is not monitoring and will not monitor the communications of the chancellor,” White House spokesman Jay Carney said on Wednesday.
‘Breach of trust’
The US has been on the receiving end of anger from allies over spying allegations based on material said to originate from fugitive American leaker Edward Snowden.
Mr Carney told reporters that Washington was examining concerns from Germany as well as France and other American allies over US intelligence practices.
But the spokesman did not address whether Mrs Merkel’s phone had been monitored in the past.
The BBC’s Nick Bryant in Washington says this is not the first time that Angela Merkel has complained about America’s alleged spying – when President Obama visited Germany in June she demanded answers.
But our correspondent says this is a much more sharply worded rebuke, and is a further sign that the diplomatic backlash over the US National Security Agency’s electronic eavesdropping is getting fiercer by the day.
Mrs Merkel’s call comes a day after US intelligence chief James Clapper denied reports that American spies had recorded data from 70 million phone calls in France in a single 30-day period.
He said a report in Le Monde newspaper had contained “misleading information”.
The German government would not elaborate on how it received the tip about alleged US spying on its leader’s communications.
US allies on spying claims
- US agencies accused of spying on leaders of Brazil and Mexico; Brazil’s Dilma Rousseffcancels state visit, Mexico’s Enrique Pena Nieto says US has promised an inquiry
- US allegedly runs bugging operations on EU mission in Washington and other European embassies; France objects, Germany cancels surveillance agreement with US and UK
- Le Monde claims NSA snooped on millions of phone calls in France; Paris complains to Washington
But news magazine Der Spiegel, which has published stories based on material from Edward Snowden, said the information had come from its investigations.
Berlin demanded “an immediate and comprehensive explanation” from Washington about what it said “would be a serious breach of trust”.
In a statement, it 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 statement also said that Mrs Merkel had told Mr Obama: “Such practices must be prevented immediately.”
A number of US allies have expressed anger over the Snowden-based spying allegations.
Brazilian President Dilma Rousseff cancelled a visit to the US this month in protest at alleged electronic espionage by the NSA against her country, including of communications at her office.
In a speech at the United Nations, she rejected arguments put forward by the US that the interception of information was aimed at protecting nations against terrorism, drugs trafficking and other organised crime.
The Mexican government has called the alleged spying on the emails of two presidents, Enrique Pena Nieto, the incumbent, and Felipe Calderon, as “unacceptable”.
US officials have begun a review of American intelligence gathering amid the international outcry.