Furious German officials said Wednesday that U.S. intelligence agencies may have been monitoring German Chancellor Angela Merkel’s cellphone, and the German leader spoke to President Obama on Wednesday about the issue, according to her spokesman.

Merkel told Obama that, if the accusations are confirmed, she “unequivocally disapproves of such practices and sees them as completely unacceptable,” her spokesman, Steffen Seibert, said in a highly unusual late-night statement.

“Between close friends and partners … there should be no such monitoring of the communication of a head of government. That would be a grave breach of trust,” Seibert said, calling for any monitoring to halt immediately.

Obama on Wednesday assured Merkel, an angry ally since the extent of the American surveillance program was disclosed several months ago, that the United States is not eavesdropping on her telephone calls after reports in Germany raised fears that such spying was taking place, the White House said Wednesday.

White House Press Secretary Jay Carney told reporters that Obama told Merkel during the phone call – initiated by the German chancellor – that the United States "is not monitoring and will not monitor" her phone conversations. Asked whether the statement left open the possibility that the NSA has swept up Merkel's calls in the past, Carney said he did not have an answer to that question.

Obama visited Germany in June and met with Merkel, who expressed displeasure at the time that the NSA had been monitoring the communications of German citizens. The new disclosures revived tensions the White House had hoped had been resolved.

The accusations were prompted by reporting by the German newsmagazine Der Spiegel, which has run many of the leaks from the National Security Agency that were released by former NSA contractor Edward Snowden. Der Spiegel said on its Web site Wednesday that the German intelligence agencies had, after a review of the magazine’s technical information, deemed the information plausible enough to confront the U.S. government about it.

Der Spiegel did not report further details about what type of monitoring may have taken place, or when, although it said that Merkel may have been the target of monitoring “for years.”

Until now, the harshest diplomatic blowback over revelations from Snowden’s leaks has come from Brazil, where President Dilma Rousseff postponed a state visit to Washington after allegations were published that the NSA had been monitoring the communications of her and her aides. The trip was to have begun Wednesday.

Merkel is a famously avid sender of text messages, and she is frequently photographed checking her smartphone during long sessions of parliament and on the road. Earlier this year, she posed at a technology fair with a secured version of a Blackberry Z10; on the back of the phone there was a decal of the black eagle that is the emblem of modern Germany.

Condemnation of the United States came swiftly from all sides of the German political spectrum Wednesday, where the news broke late in the evening.

“If the accusations are true, that would be a serious breach of trust. Therefore this needs to be cleared up very quickly,” said Thomas Oppermann, a member of the opposition Social Democrats who has been leading a parliamentary investigation into whether German intelligence agencies had collaborated with the NSA to illegally obtain information about German citizens. He spoke in an interview with German broadcaster ARD.

Revelations of NSA spying in Germany had caused major political uproar in the country earlier this year, with investigations and fallout lasting long after outrage over Snowden’s revelations had settled in the United States. Earlier this month, Deutsche Telekom, Germany’s largest Internet and telephone provider, proposed routing important German e-mail traffic exclusively within German networks to ensure that it is secure from U.S. intelligence agencies.

Local press have also reported that a domestically modified, highly secure version of the Blackberry Z10 smartphone has been selling out in the months since the NSA revelations were first published, driven by high demand from German government agencies.

And European Union policymakers in Brussels gave preliminary approval to a stringent new set of data protection regulations that would severely restrict the kind of information that private companies such as Facebook and Google can hand over to governments about E.U. users.

Alleged U.S. spying also caused a diplomatic uproar in France this week, when Le Monde reported that the NSA had tapped millions of phones in that country and that it had also been monitoring the electronic communications of French diplomats. That reporting was also based on leaks from Snowden.

On Wednesday, Director of National Intelligence James R. Clapper denied Le Monde’s report, saying in a statement that it contained “inaccurate and misleading information” and specifically calling “false” the allegation that the NSA collected more than 70 million recordings of French citizens’ telephone data.

“We have repeatedly made it clear that the United States gathers intelligence of the type gathered by all nations,” Clapper said in the statement.

Michael Birnbaum is reporting from Berlin. Ellen Nakashima contributed to this report.