US President Barack Obama was "personally informed" of the surveillance of German Chancellor Angela Merkel's mobile phone, which may have begun as early as 2002, the German media reported on Sunday.

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

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

In a separate report, German newsweekly Der Spiegel said ahead of its Monday issue, leaked NSA documents showed Merkel's phone was on a list of spying targets by the US since 2002, and was being tapped even weeks before Obama visited Berlin in June.

Germany said it will send its own intelligence officials to the US next week to seek answers after the media reports surfaced, in a move that threatened to fray ties between the two nations.

Merkel confronted Obama with the suspicion over telephone on Wednesday, when she reportedly told him that spying on allies would be treated as a "breach of trust," if proved by evidence.

In a highly unusual move, Berlin also summoned US ambassador John B Emerson to register its protest.

After Merkel's conversation with Obama, Der Spiegel had reported that the president had told Merkel that if he had been informed of the operation, he would have stopped it.

Another German newspaper, the Frankfurter Allgemeine Sonntagszeitung, 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.

The spying row prompted several European leaders to demand a new deal with Washington on intelligence gathering that would help them maintain an "essential alliance" while not taking the eye off their "fight against terrorism."

Other German media reports said that Obama's National Security Advisor Susan Rice had also told German officials the president knew nothing of the spying.

At last week's European Union summit in Brussels, the 28 leaders also warned that while the bloc and the US shared a "close relationship," it must "be based on respect and trust."

Separately, Germany and Brazil are working on a United Nations General Assembly resolution to highlight international concern over US's data snooping in other countries, diplomats said on Friday.

The resolution would not mention the US but would call for extending the International Covenant on Civil and Political Rights to cyber activities.

"The aim is to send a message to those who abuse the system," said an  UN diplomat involved in the talks.

AFP - Global Times