The imbroglio over the tapping of Angela Merkel's phone deepened Sunday, after a US denial that President Barack Obama was informed years ago of electronic surveillance of the German chancellor.

As a sense of betrayal spread in European capitals about spying activities conducted on world leaders and ordinary citizens, German media reports said eavesdropping on Merkel's phone may have begun as early as 2002.

The Bild am Sonntag newspaper quoted US intelligence sources as saying that America's National Security Agency chief General Keith Alexander had specifically briefed Obama on the operation against Merkel in 2010.

"Obama did not halt the operation but rather let it continue," the newspaper quoted a high-ranking NSA official as saying.

News weekly Der Spiegel reported that leaked NSA documents showed Merkel's phone had appeared on a list of spying targets for over a decade, and was still under surveillance weeks before Obama visited Berlin in June.

But NSA spokeswoman Vanee' Vines, in Washington, flatly denied the claims. 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," Vines said.

"News reports claiming otherwise are not true," she added. The allegations, derived from documents acquired from US fugitive defense contractor Edward Snowden, have stoked global outrage that American spy agencies were responsible for broad snooping into the communications of several dozen world leaders and likely millions of foreign nationals.

A poll for Der Spiegel found that 60 percent of Germans believe the scandal has damaged bilateral ties, though on Sunday US lawmakers sought to play down the allegations.

Representative Peter King, who chairs the House Subcommittee on Counterterrorism and Intelligence, said Obama should "stop apologizing" about the NSA's phone-tapping scandal, claiming the programs had saved "thousands" of lives.

And House Intelligence Committee Chairman Mike Rogers told CNN: "The bigger news story here would be... if the United States intelligence services weren't trying to collect information that would protect US interests both (at) home and abroad."

Germany, however, is to send its own spy chiefs to Washington to demand answers. Swiss President Ueli Maurer warned the revelations risked "undermining confidence between states."

"We don't know if we're only seeing the tip of the iceberg or if other governments are acting in the same ruthless manner," he told the Schweiz am Sonntag weekly.

With anger simmering in Berlin, Interior Minister Hans-Peter Friedrich sharpened his tone.

"Surveillance is a crime and those responsible must be brought to justice," he told Bild, while Foreign Minister Guido Westerwelle decried the "highly damaging" spying among friends.

Merkel confronted Obama with the snooping allegations in a phone call Wednesday saying that such spying would be a "breach of trust."

The Frankfurter Allgemeine Sonntagszeitung said Obama had told Merkel during their call that he had been unaware of any spying against her, while Spiegel said he assured her that he would have stopped the operation at once.

Merkel's office declined to comment on what Obama told her. The White House has said it is not monitoring Merkel's phone calls and will not do so in future, but it has refused to say whether it did previously.