The German government has received such information and Berlin has asked Washington for an "immediate and comprehensive explanation," spokesman Steffen Seibert said.
Merkel spoke by telephone with US President Barack Obama Wednesday and "made clear that she unequivocally condemns and regards such practices as fully unacceptable if the information is proven true," Seibert said.
Obama assured Merkel that US agencies were not monitoring her mobile telephone communications, the White House said.
He said, "The US is not monitoring and will not monitor communications of the chancellor," spokesman Jay Carney said.
The phone call was made a day before Merkel was expected in Brussels for an EU summit, at which EU leaders were expected to discuss data protection in the wake of claims of widespread US monitoring of internet traffic and phone calls.