German Chancellor Angela Merkel has called United States President Barack Obama after receiving information the Americans may have spied on her mobile phone.
A German government spokesperson says Mrs Merkel sought immediate clarification of the matter in a phone call with Mr Obama, and made it clear any such monitoring would be a serious breach of trust.
The White House says Mr Obama told Mrs Merkel the United States is not monitoring her communications.
"The United States is not monitoring and will not monitor the communications of the chancellor," White House spokesman Jay Carney said on Wednesday.
The US has been on the receiving end of anger from allies over spying allegations based on material said to originate from fugitive American leaker Edward Snowden.
The BBC reports this is a much more sharply worded rebuke from Ms Merkel than previous comments she has made on alleged US spying, and is a sign that the diplomatic backlash over US eavesdropping is getting fiercer.