US' NSA tracked Angela Merkel's phone since 2002

The United States reportedly tracked German chancellor Angela Merkel's phone for more than 10 years.

A report in the German Magazine Der Spiegel also said that US President Barack Obama told Merkel he would have stopped it happening had he known about it.

Germany was outraged over reports that Merkel's phone was bugged by the US National Security Agency (NSA).

Germany summon the US ambassador this week for the first time, an unprecedented post-war diplomatic rift.

Spiegel said Merkel's mobile telephone had been listed by the NSA's Special Collection Service (SCS) since 2002, marked as 'GE Chancellor Merkel', and was still on the list weeks before Obama visited Berlin in June.

In an SCS document cited by the magazine, the agency said it had a 'not legally registered spying branch' in the US embassy in Berlin.

From there, NSA and CIA staff were tapping communication in the Berlin's government district with high-tech surveillance.

Quoting a secret document from 2010, Der Spiegel said such branches existed in about 80 locations around the world, including Paris, Madrid, Rome, Prague, Geneva and Frankfurt.

The magazine said it was not clear whether the SCS had recorded conversations or just connection data.

Obama apologised to Merkel when she called him on Wednesday to seek clarification on the issue.