US spying agencies have been monitoring German Chancellor Angela Merkel's mobile phone since 2002, Der Spiegel magazine reported on Saturday, dpa reported.
It added that US President Barack Obama told the German leader that he would have stopped the eavesdropping had he known about it.
Revelations that Merkel's phone was being tracked by the US National Security Agency (NSA) have sparked an outcry in Germany, and prompted the government to summon the US ambassador. German intelligence officials were to travel to Washington to meet with their US counterparts in the coming weeks, the governments said Friday.
Der Spiegel said Merkel's mobile phone number appeared under "GE Chancellor Merkel" on documents of the NSA's Special Collection Service.
It said the Special Collection Service was operating a "not legally registered espionage branch" at the US embassy in Berlin, where highly advanced equipment was being used to spy on the German government.
Based on a secret 2010 document, the magazine added that the NSA was operating similar spying centres in some 80 location worldwide, including 19 in Europe. It named Paris, Madrid, Rome, Prague, Geneva and Frankfurt as homes to such NSA branches.
The German government said on Friday that Merkel's calls were safe given she normally uses encrypted phones.
Mounting revelations about the digital surveillance activities of the NSA - many of which originated with fugitive whistleblower Edward Snowden - have strained US relations with European allies.
Germany and Brazil are expected to introduce a resolution at the United Nations next week calling for expansion of privacy rights online and seeking the UN human rights commissioner to take up the issue.
Merkel called Obama this week to complain about the revelations.
In Washington, several hundred demonstrators marched to the US Capitol to protest government surveillance programmes for the Stop Watching Us rally.
Some protesters carried large, cardboard models of mobile phones in an apparent reference to the most recent allegations that the NSA spied on the phones of dozens of world leaders.
The rally was organized by some 100 groups from across the political spectrum to call for greater transparency in the US spying programme and for stricter privacy protections. They were to deliver a petition with more than 570,000 signatures to Congress.