The U.S. National Security Agency may have bugged German Chancellor Angela Merkel's phone for more than 10 years, according to a news report Saturday by the German weekly Der Spiegel.
Der Spiegel also cited a source in Ms. Merkel's office saying U.S. President Barack Obama apologized to the German leader when she called him this past Wednesday to seek clarification on the issue.
A Merkel spokesman and the White House declined comment.
Germany plans to send its intelligence chiefs to Washington in the coming days to seek answers on the spying allegations.
Documents leaked by former NSA contractor Edward Snowden that show sweeping U.S. surveillance on Internet searches and telephone records of ordinary citizens and world leaders have sparked outrage globally.
Germany is also working with Brazil on a draft United Nations General Assembly resolution to guarantee people's privacy in electronic communications. U.N. diplomats say it would call for extending the International Covenant on Civil and Political Rights to Internet activities, but would not mention the United States.
As many as 1,000 demonstrators rallied Saturday outside the U.S. Capitol against NSA spying, demanding an end to mass surveillance of individuals.
President Obama has ordered a review of U.S. surveillance programs after Snowden leaked the NSA secrets.
Former CIA deputy director Michael Morrell said in a television interview to be broadcast Sunday that Snowden's leaks are "the most serious compromise of classified information in the history of the U.S. intelligence community."
Morrell told CBS television's 60 Minutes the most damaging leaked document was the so-called Black Budget, detailing where the U.S. spends its money on intelligence efforts.
Morrell said Snowden has put Americans at greater risk "because terrorists learn from leaks and they will be more careful," and the country will not get the intelligence it would have gotten otherwise.
The Washington Post reported Friday U.S. officials are warning some foreign intelligence services that documents obtained by Snowden detail their secret cooperation with Washington.