The German government has obtained information that the United States may have monitored the mobile phone of Chancellor Angela Merkel.
She called President Barack Obama to demand an immediate clarification, a German government spokesman said this evening.
"We swiftly sent a request to our American partners asking for an immediate and comprehensive clarification," Steffen Seibert said in a statement.
He added that Ms Merkel had made clear to Mr Obama that if the information proved trued it would be "completely unacceptable" and represent a "grave breach of trust".
President Obama moved to assure Ms Merkel that the US is not monitoring her communications.
White House spokesman Jay Carney said Mr Obama and Ms Merkel spoke on the phone about German fears that the US National Security Agency had been spying on her.
Mr Carney, asked about the allegation that the NSA had monitored her mobile phone conversations, said Mr Obama assured Ms Merkel that "the United States is not monitoring the communications of the chancellor".