Image caption President Obama is reported to have apologised to Chancellor Merkel

The chief of the US spy agency NSA has not discussed the alleged bugging of German chancellor's phone with President Barack Obama, officials say.

Gen Keith Alexander never discussed alleged operations involving Chancellor Angela Merkel, an NSA spokeswoman said.

German media say the US has been tapping the chancellor's phone since 2002, and Mr Obama was told in 2010.

The row has led to one of the worst diplomatic crises between the two countries in recent years.

A report in German tabloid Bild am Sonntag claimed that Gen Alexander had told the president about the bugging himself.

Analysis By all accounts, Angela Merkel has been genuinely shocked by the revelations. People close to her told the BBC she felt personally affronted. When Barack Obama was in Berlin in June, they did seem to get on well. She is not good at hiding her feelings, and the glum scowl she used to reserve for Silvio Berlusconi, for example, was replaced by a beam of warmth. They were tactile - he would put his arm round her back; she would clutch his elbow. Perhaps the sense of betrayal is all the greater because of her background in the East German communist regime where spying was pervasive. She might have expected it from the Stasi but not from her new best friend. Others might feel betrayed, too. When the original allegations of widespread phone-tapping emerged, some of Chancellor Merkel's confidantes belittled the problem, saying the criticism of the US had a touch of anti-Americanism and that the surveillance was about terrorism. These people are now some of the strongest critics of the US. They are also saying that German law has been broken. If the activities of American government employees were investigated by the German authorities, that would make the whole affair harder to damp down. It would be in the system of justice and pursuit would be relentless. Germany's texting Chancellor Merkel German papers lay into Obama

An NSA source told the paper that Obama had not stopped the operation, and had wanted to know all about Mrs Merkel as "he did not trust her".

'Not true'

However a statement from the National Security Agency on Sunday denied the reports in Bild.

"[General] Alexander did not discuss with President Obama in 2010 an alleged foreign intelligence operation involving German Chancellor Merkel, nor has he ever discussed alleged operations involving Chancellor Merkel," NSA spokeswoman Vanee Vines said.

"News reports claiming otherwise are not true."

The statement does not make it clear whether the president was informed of the bugging operation by other means.

Mr Obama is reported to have told the German chancellor that he knew nothing of the operation when the two leaders spoke.

Germany is sending its top intelligence chiefs to Washington in the coming week to "push forward" an investigation into the spying allegations, which have caused outrage in Germany.

Der Spiegel's report, based on leaked documents, says a US listening unit was based in its Berlin embassy - and similar operations were replicated in 80 locations around the world.

Media playback is unsupported on your device Media caption Angela Merkel: "Once the seeds of mistrust have been sown it doesn't facilitate our co-operation... it makes it more difficult"

Such a listening post would be illegal under German law, according to Germany's interior minister.

And the documents seen by the magazine suggest the US was aware of the sensitivities of siting listening stations in US embassies. If their existence were known, they say, there would be "severe damage for the US's relations with a foreign government".

A unit called Special Collection Services, based on the fourth floor of the US embassy in Pariser Platz in Berlin, was responsible for monitoring communications in the German capital's government quarter, including those targeting Mrs Merkel it said.

Extensive bugging?

Der Spiegel says the NSA documents show Mrs Merkel's number on a list dating from 2002 - three years before she became chancellor.

Image caption Mrs Merkel was awarded the US Presidential Medal of Freedom in 2011

This might indicate that there was extensive bugging of the phones of prominent people, says the BBC's Stephen Evans in Berlin.

The nature of the monitoring of Mrs Merkel's mobile phone is not clear from the files, Der Spiegel says.

For example, it is possible that the chancellor's conversations were recorded, or that her contacts were simply assessed.

Mrs Merkel phoned President Barack Obama when she first heard of the spying allegations on Wednesday.

The president apologised to the German chancellor, Der Spiegel reports.

Mrs Merkel - an Americophile who was awarded the US Presidential Medal of Freedom in 2011 - is said to be shocked that Washington may have engaged in the sort of spying she had to endure growing up in Communist East Germany.