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

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

Bild am Sonntag newspaper quoted US intelligence sources as saying that America's National Security Agency chief General Keith Alexander had 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 ordinary people.

A poll for Der Spiegel found that 60 percent of Germans believe the scandal has damaged bilateral ties.

European leaders have since called for a new deal with Washington on intelligence gathering that would maintain an essential alliance while keeping the fight against terrorism on track.

Germany 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.

Two phones monitored

Bild said Obama wanted to be informed in detail about Merkel, who has played a decisive role in the eurozone debt crisis and is widely seen as Europe's most powerful leader.

As a result, the NSA stepped up its surveillance of her communications, targeting not only the mobile phone she uses to conduct business for her conservative Christian Democratic Union party but also her encrypted official device.

Merkel only acquired the latter handset over the summer.

Bild said US specialists were then able to monitor the content of her conversations as well as text messages, which Merkel sends by the dozen each day to key associates.

Only the specially secured land line in her office was out of the reach of the NSA, which sent the intelligence gathered straight to the White House bypassing the agency's headquarters, according to the report.

Bild and Spiegel described a hive of spy activity on the fourth floor of the US embassy in central Berlin, a stone's throw from the government quarter, from which the United States kept tabs on Merkel and other German officials.

Spiegel cited a classified 2010 document indicating that US intelligence had 80 high-tech surveillance offices worldwide in cities including Paris, Madrid, Rome, Prague, Geneva and Frankfurt.

If the spying against Merkel began in 2002, it would mean the United States under then president George W. Bush targeted her while she was still the country's chief opposition leader, three years before she became chancellor.

Bild said Merkel's predecessor Gerhard Schroeder was also in the NSA's sights because of his vocal opposition to the US invasion of Iraq and close relationship with Russian President Vladimir Putin.

The Social Democrats' chief whip Thomas Oppermann told Bild that German MPs would now like to question Snowden in a new parliamentary probe of the affair.

"Snowden's accounts seem credible while the US government apparently lied to us about this matter," he said.