US President Barack Obama has assured German Chancellor Angela Merkel that the US is not monitoring her communications, according to the White House spokesman.
Jay Carney's statement came on Wednesday in response to the German government's announcement that it has obtained information suggesting the US may have tapped Merkel's mobile phone.
Merkel called Obama to demand an immediate clarification, her spokesman said in a statement.
"She made clear that she views such practices, if proven true, as completely unacceptable and condemns them unequivocally," the statement read.
"Between close friends and partners, as Germany and the US have been for decades, there should not be such monitoring of the communications of a government leader. This would be a grave breach of trust. Such practices should be immediately stopped."
'Shared security challenges'
For its part, the White House denied that the US is listening in on Merkel's phone calls now.
"The president assured the chancellor that the United States is not monitoring and will not monitor the communications of the chancellor," Carney said.
"The United States greatly values our close cooperation with Germany on a broad range of shared security challenges."
However, Carney did not specifically say that the US had never monitored or obtained Merkel's communications.
The news broke as John Kerry, US secretary of state, faced fresh questions during a visit to Rome about mass spying on European allies, based on revelations from Edward Snowden, the fugitive ex-US intelligence operative granted asylum in Russia.
French President Francois Hollande is pressing for the US spying issue to be put on the agenda of a summit of European leaders starting on Thursday.
French newspaper Le Monde reported earlier this week that the National Security Agency had collected tens of thousands of French phone records.
Just four months ago, Obama defended US anti-terrorism tactics on a visit to Berlin, telling Germans at a news conference with Merkel that the US was not spying on ordinary citizens.