Germany has dropped an investigation into alleged tapping of Chancellor Angela Merkel's phone by the U.S. National Security Agency (NSA), according to BBC.

The office of federal prosecutor Harald Range said the NSA had failed to provide enough evidence to justify legal action.

The allegations of NSA phone-tapping came out in the secrets leaked by U.S. whistleblower Edward Snowden about large-scale U.S. surveillance in 2013.

German-U.S. ties were severely strained.

When the allegations were made the White House gave no outright denial, but said Merkel's phone was not being bugged currently and would not be in future.

On June 4, last year Range said "sufficient factual evidence exists that unknown members of the US intelligence services spied on the mobile phone of Chancellor Angela Merkel".

But in December he revealed that the investigation was not going well and he had not obtained enough evidence to succeed in court.