Germany has closed its investigation into a report that the US National Security Agency had hacked Chancellor Angela Merkel's mobile phone, a move that appears to be aimed at ending transatlantic friction that threatened intelligence cooperation between the two countries.

US intelligence agencies have been angered by the amount of sensitive information being made public as a result of German investigations into US surveillance after the Edward Snowden revelations two years ago.

NSA listed Merkel among leaders subject to surveillance – report

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German federal prosecutors announced on Friday that their investigation was being wound down because they had been unable to find evidence that would stand up in court.

The investigation came after Der Spiegel reported in October 2013 that the NSA had a database containing Merkel's personal phone number. Merkel publicly expressed outrage and dispatched a team of senior German intelligence officers to Washington, supposedly to extract a "no spy" agreement. When the row was its height, the chancellor said: "The charges are grave and have to be cleared up."

A German federal investigation began last June but the office of the German chief prosecutor, Harald Range, announced on Friday that it did not have an original NSA document proving the NSA spied on Merkel.