The German government's spokesman said it believes Chancellor Angela Merkel's mobile phone may have been monitored by the National Security Agency, Der Spiegel reports.
Merkel has complained to Obama directly.
The White House said that Obama and Merkel spoke by phone Wednesday, and he " assured " Merkel that the U.S. "is not monitoring and will not monitor" her phone calls. They didn't comment on what was done in the past.
Merkel said that targeting her phone would be unacceptable.
"The United States greatly values our close cooperation with Germany on a broad range of shared security challenges," the White House said in a readout of the leaders' call.
"As the President has said, the United States is reviewing the way that we gather intelligence to ensure that we properly balance the security concerns of our citizens and allies with the privacy concerns that all people share.
"Both leaders agreed to intensify further the cooperation between our intelligence services with the goal of protecting the security of both countries and of our partners, as well as protecting the privacy of our citizens."
The report comes after allegations that t he NSA read text messages and listened to phone calls of President Enrique Pena Nieto and recorded 70.3 million French civilian phone calls from Dec. 10, 2012, to Jan. 8 of this year.
The previous revelations came from the trove of classified data taken from NSA systems by Edward Snowden, who is currently residing in Russia.
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