The United States has been monitoring German Chancellor Angela Merkel’s telephone conversations for more than 10 years, German weekly Der Spiegel said in a report published on Saturday.

The United States has been monitoring German Chancellor Angela Merkel's telephone conversations for more than 10 years, German weekly Der Spiegel said in a report published on Saturday.

The report said that the magazine had seen secret documents from the US National Security Agency (NSA) which show that Merkel's mobile phone had been listed by the agency’s Special Collection Service (SCS) since 2002. 

The report adds that the German chancellor’s mobile number was still on a surveillance list in June 2013. 

On Friday, the German government said its intelligence chiefs would visit Washington soon to probe the spying controversy. 

Government spokesperson Georg Streiter said that the heads of Germany's foreign and domestic intelligence would hold meetings with the officials from the White House and the NSA. 

He did not provide an exact date for the trip, saying it was being set on "relatively short notice." 

“What exactly is going to be regulated, how and in what form it will be negotiated and by whom, I cannot tell you right now,” he stated. “But you will learn about it in the near future because we have put some pressure to do this speedily.” 

Meanwhile, US National Security Council spokesperson Caitlin Hayden said the Germans would be welcomed, but did not explain how Washington would deal with the anger felt by the European country over tapping its leader’s phone conversations. 

The Guardian reported on Thursday that the NSA had monitored the telephone conversations of 35 world leaders. 

"A US official provided NSA with 200 phone numbers to 35 world leaders … Despite the fact that the majority is probably available via open source, the PCs [intelligence production centers] have noted 43 previously unknown phone numbers. These numbers plus several others have been tasked," according to a classified document provided by US whistleblower Edward Snowden. 

"These numbers have provided lead information to other numbers that have subsequently been tasked," it added.