Report: NSA spying on German ministers instead of Merkel

Among the officials said to be targeted is interior minister Thomas de Maiziere, Merkel's former chief of staff and close ally.

Merkel often calls de Maiziere ahead of key decisions and once asked him "what should I think" - to the surprise of the people listening in on her conversation, Bild reported.

De Maiziere, who served as defence minister until the new government was formed last year, was reportedly floated as potential successor to Nato secretary general Anders Fogh Rasmussen.

This was allegedly also a reason for tapping his phone. "We wanted to know if he is a reliable partner," the NSA source said.

The German government refused to comment on the news report.

In total, some 320 high-ranking officials and businessmen are said to be on the NSA watchlist. The German software company SAP, a competitor to US firm Oracle, is reportedly also being spied upon by the NSA.

Bild am Sonntag quoted Obama's security adviser Caitlin Hayden denying that Washington is spying on German companies in order to help American firms.

"The United States has made clear it gathers intelligence in exactly the same way as any other states," Hayden said.

The news that the NSA was tapping Merkel's phone and that of Chancellor Gerhard Schroeder before her has upset German-US relations.

Obama in January promised to halt eavesdropping on allied leaders, but has so far rejected Merkel's push for a "no-spy agreement" between the two countries.

Foreign minister Frank-Walter Steinmeier is due to travel to Washington on Thursday, but he told Der Spiegel he has "doubts that a no-spy agreement will get us much further."

"I hope Washington understood that the way it deals with partners can also have a political price," Steinmeier added.

Merkel herself has been invited to the US capital to mend ties with Obama, but she has not yet confirmed a date.