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 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.
Charges shake US alliances
Germany's leader has said trust between her country and the United States, a close ally, needs to be rebuilt, and French President Francois Hollande has also been a vocal critic of the NSA's gathering phone and email records.
But it's not just European leaders who object to the alleged spying.
Former NSA contractor Edward Snowden has revealed classified documents that show as many as 35 world leaders' cell phones were tapped.
Brazilian President Dilma Rousseff canceled a state visit to Washington and condemned the U.S. in a speech to the United Nations in September, saying "the safety of a citizen of one country should not be guaranteed by the violation of human and civil rights which are fundamental to citizens of another country."
While many countries spy on each other, Scheherazade Rehman, head of George Washington University's European Union Center, says this situation is unusual.
"These kinds of things go on between countries, but to have it so overtly splashed on the front pages is something that outrages most countries, I think."
Rehman does not foresee a long-term chill in U.S. relations with its allies, but she says the NSA will need to change some procedures.
"I think specifically targeting information that we are going after, as opposed to a general gathering of information, that needs to be discussed."
NSA Director Keith Alexander said recently that despite the public objections, many U.S. allies tell him they welcome the intelligence gathering.
"Here is what I get: 'Keep working with us — the intelligence you get us to defend our country is what we really need,'" he said.
But State Department spokeswoman Jen Psaki said Friday that the issue has complicated America's relations with its allies, and "created significant challenges in our relationships with some of our partners, and has been, of course, a public distraction, as you even saw over the last couple of days."
The challenges in those relationships will also be a challenge to policymakers in Washington, who are reluctant to give up any weapon in the fight against terrorism.
VOA correspondent Kent Klein contribued to this report, and some information was provided by AFP.