The chancellor's mobile phone number was on a list of NSA monitoring targets found in classified documents leaked by former NSA contractor Edward Snowden.
The documents revealed that the spy agency started its surveillance of Ms Merkel's mobile phone in 2002 when George Bush was in the White House, weekly news magazine Der Spiegel said in its latest edition.
The NSA's assignment for spying on the chancellor was valid even some weeks before President Barack Obama visited Berlin in June.
The agency's list carried Ms Merkel's mobile phone number under "GE Chancellor Merkel", the news weekly said.
The agency's European desk S2C32 "European States Branch" identified Ms Merkel as a "target" for surveillance, according to the documents.
A unit called Special Collection Services (SCS) was responsible for carrying out the order.
Der Spiegel said it was not clear from the documents whether the snooping agency stored her conversations or only the connecting data were evaluated.
The Special Collection Services kept a secret espionage centre at the American embassy, which is around 600 metres away from the chancellery and closer to the Reichstag building housing the German parliament, the documents revealed.
Officials of the NSA and the Central Intelligence Agency (CIA), who jointly operated from the centre, systematically monitored the communications in Germany's centre of power  with highly sophisticated eavesdropping techniques, the news weekly said.
The NSA documents showed its staff were warned in an internal "top secret" SCS notification that if diplomatic missions with their high-performance antenna and other equipment were detected as espionage centres, it could cause "severe damage" to America's relations with a foreign government.
Besides the embassy in Berlin, the US government maintained a second German espionage centre in Frankfurt, Spiegel said, quoting the NSA internal documents.
Similar joint operational centres of the CIA and the NSA, which were established in the late 1970s, exist in 80 locations worldwide.
Among them are 19 in European cities such as Paris, Rome, Madrid, Geneva and Prague, according to a secret list of locations from 2010, Der Spiegel said.


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