Germany has summoned the US ambassador in Berlin over claims that the US monitored German Chancellor Angela Merkel's mobile phone.
Foreign Minister Guido Westerwelle will meet US envoy John Emerson later in what is seen as an unusual step between close allies.
Mrs Merkel has demanded a "complete explanation" of the claims, which are threatening to overshadow an EU summit.
She discussed the issue with US President Barack Obama on Wednesday.
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