The Obama administration appeared in disarray as it struggled with the fallout over the disclosure that the National Security Agency monitored the phone conversations of at least 35 world leaders, and that the phone of the German chancellor, Angela Merkel, had been monitored.

Early on Sunday, the White House refused to comment on an overnight report in the German tabloid Bild, which alleged that Obama was personally briefed about by the operation to target Merkel's phone by the NSA's director, Keith Alexander, and allowed it to continue.

That appeared to conflict with a second report, in the German newspaper Frankfurter Allgemeine Sonntagszeitung. It said that when Obama spoke to Merkel over the phone on Wednesday, he assured the German leader he had not previously known her phone had been monitored.

Caitlin Hayden, the White House's national security council spokeswoman, declined to comment on the reports, telling the Guardian: "We are not going to comment publicly on every specific alleged intelligence activity."

However just over three hours later, that position appeared to have been reversed, when the NSA said in a statement that 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. News reports claiming otherwise are not true".

Bild cited "a secret intelligence employee who is familiar with the NSA operation against Merkel" in its story, which was widely-picked up by global media.

The intervention by the NSA was unusual – the agency rarely comments publicly on specific allegations about its surveillance activities.

However the White House and NSA are coming under intense pressure to reveal the extent to which Obama and senior administration officials knew about US surveillance operations targeting the leaders of allied countries.

The agency did not dispute another report in Germany, based on documents disclosed by the whistleblower Edward Snowden, that the NSA's Special Collection Service (SCS) had listed Merkel's phone number since 2002. The report, in Der Spiegel, said the phone number was still on the list – marked as "GE Chancellor Merkel" – weeks before Obama visited Berlin in June, raising the possibility that the German leader had been under surveillance for more than a decade.

In an SCS document cited by the magazine, the agency said it had a "not legally registered spying branch" in the US embassy in Berlin, the exposure of which would lead to "grave damage for the relations of the United States to another government".

A delegation of German intelligence officials are due to arrive in the US to meet counterparts in the coming days.
They are expected to demand clarity on the nature and scope of NSA activities in their country.

SHI/SHI