A German newspaper has reported that US President Barack Obama knew his intelligence service was eavesdropping on Angela Merkel as long ago as 2010, contradicting reports that he had told the German chancellor he did not know.
Germany received information this week that the US National Security Agency (NSA) had bugged Merkel's mobile phone, prompting the German government to summon the US ambassador.
The NSA denied that Obama had been informed about the operation by the NSA chief in 2010, as reported by the German paper Bild am Sonntag on Sunday.
But the agency did not comment directly on whether Obama knew about the bugging of Merkel's phone.
Both the White House and the German government declined comment.
The Wall Street Journal reported on Sunday that the NSA ended the programme that involved Merkel after the operation was uncovered in an Obama administration review that began this summer.
The programme also involved as many as 35 other world leaders, some of whom were still being monitored, according to the report, which was attributed to US officials.
Caitlin Hayden, US National Security Council spokeswoman, said in a statement that Obama had ordered a review of US surveillance capabilities.
Citing a source in Merkel's office, some German media have reported that Obama apologised to Merkel when she called him on Wednesday, and told her that he would have stopped the bugging happening had he known about it.
But Bild am Sonntag, citing a "US intelligence worker involved in the NSA operation against Merkel", said General Keith Alexander, NSA chief, informed Obama in person about it in 2010.
"Obama didn't stop the operation back then but let it continue," the newspaper quoted the source as saying.
The NSA said, however, that Alexander had never discussed any intelligence operations involving Merkel with Obama.
"[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", Vanee Vines, NSA spokeswoman, said in an emailed statement.
"News reports claiming otherwise are not true."
The Frankfurter Allgemeine Sonntagszeitung, another German paper, reported on Saturday that Obama had told Merkel during their call that he had been unaware of any spying against her. It did not cite its sources.
Other media reports said that Susan Rice, Obama's national security adviser, had also told German officials the president knew nothing of the spying.
But Bild am Sonntag said Obama in fact wanted more material on Merkel, and ordered the NSA to compile a "comprehensive dossier" on her.
Obama, according to the NSA man, did not trust Merkel and wanted to know everything about the German," the paper said.
White House spokeswoman declined to comment and reiterated the standard policy line that the US gathers foreign intelligence of the type gathered by all nations.
Bild am Sonntag said the NSA had increased its surveillance, including the contents of Merkel's text messages and phone calls, on Obama's initiative and had started tapping a new, supposedly bug-proof mobile she acquired this summer, a sign the spying continued into the "recent past".
Merkel has said she uses one mobile phone and that all state-related calls are made from encrypted lines.
Germany will send its own spy chiefs to the US next week to demand answers following the allegations.
The NSA first eavesdropped on Merkel's predecessor Gerhard Schroeder after he refused to support President George W Bush's war in Iraq and was extended when Merkel took over in 2005, Bild am Sonntag said.
Eighteen NSA staff working in the US embassy, just 800 metres from Merkel's office, sent their findings straight to the White House, rather than to NSA headquarters, the paper said.
Only Merkel's encrypted landline in her office in the chancellery had not been tapped, it added.
Bild said some NSA officials were becoming annoyed with the White House for creating the impression that US spies had gone beyond what they had been ordered to do.
The rift over US surveillance activities first emerged this year with reports that the US had bugged European Union offices and tapped half a billion phone calls, emails and text messages in Germany in a typical month.