The White House has refused to say whether the United States has spied on German Chancellor Angela Merkel, amid a growing furore over allegations Washington tapped her mobile phone.
Press secretary Jay Carney says the White House is "not going to get into specific allegations that have been made in public reports."
On Wednesday, US President Barack Obama had assured Merkel the United States was not currently spying on her, and would not in the future, but Carney left open whether such spying had occurred in the past.
In Berlin, meanwhile, Germany's foreign minister summoned the US ambassador for an explanation, as outrage over the alleged spying grew.
Carney says US discussions about the allegations, which have also involved other foreign leaders, are confined to diplomatic channels.
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"When it comes to specific allegations about intelligence matters, we have diplomatic relations channels that we use in order to discuss these issues that have clearly caused some tension in our relationships with other nations around the world, and that is where we are having those discussions," he said.
