Media playback is unsupported on your device Media caption White House spokesman Jay Carney confirmed that President Obama had "assured" Chancellor Merkel that the US was not monitoring her phone

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 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.

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.

'Breach of trust'

Analysis The language in the White House statement responding to allegations that the NSA monitored Angela Merkel's private mobile phone is deliberately precise. "The president assured the chancellor that the United States is not monitoring and will not monitor the communications of Chancellor Merkel." It did not deny possible past surveillance on her phone. Clearly, Angela Merkel believes these allegations are plausible enough to confront directly Barack Obama, in what must have been an awkward conversation. Tonight at the White House there was supposed to be a state dinner for Brazilian President Dilma Rousseff. But she cancelled the visit last month following allegations that the NSA snooped on her personal communications. Only on Monday, President Obama had to reassure his French counterpart, Francois Hollande, over allegations published in Le Monde of electronic eavesdropping on French political figures and business leaders on a vast scale. The diplomatic backlash is getting fiercer by the day.

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.

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."

The BBC's Steve Evans in Berlin says because the statement was issued after the phone call, there were indications that Mrs Merkel had not been reassured.

He says the issue of state monitoring of phone calls is a real one in Germany - Angela Merkel grew up in East Germany, where phone tapping was pervasive.

President Obama had assured Chancellor Merkel when he visited in June that German citizens were not being spied upon and our correspondent says she was criticised then by political opponents for not being more sceptical.

The German government would not elaborate on how it received the tip about the alleged US spying.

But news magazine Der Spiegel, which has published stories based on material from Edward Snowden, said the information had come from its investigations.

US allies on spying claims US agencies accused of spying on leaders of Brazil and Mexico; Brazil's Dilma Rousseff cancels 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 Leaks timeline

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".

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.