The German government has asked for clarification following reports that US spy agencies may have tapped the phone of chancellor Angela Merkel.
German news magazine Spiegel claims that following its own research, it brought the matter to the attention of Germany's Federal Intelligence Service (BND) and the Federal Office for Information Security.
The paper reports Merkel made a phone call to US President Barack Obama to discuss her intelligence services' suspicions. Merkel was said to "unequivocally disapprove" of the actions, should they turn out to be true. Her spokesman Steffen Seibert added that Merkel found such methods "totally unacceptable" and a "breach of trust". "Such practices must immediately be put to a stop," the statement said.
"As a close ally of the United States of America, the German government expects a clear contractual agreement on the activities of the agencies and their cooperation," Seibert added.
A spokeswoman for the US National Security Council told Spiegel: "The President assured the Chancellor that the United States is not monitoring and will not monitor the communications of Chancellor Merkel."
The paper highlights that the spokeswoman did not specify whether the statement also applied to the past.
The long and twisting tale of the PRISM scandal has cooled diplomatic relations between the EU and the US in recent months, with previous allegations including the bugging of European Union buildings and the tapping of EU telephones.
The PRISM scandal - sparked by NSA whistle blower Edward Snowden - has also sent shockwaves across the technology industry, with major tech corporations implicated in the wholesale scanning of internet traffic and social network activity.