Germany summoned the US ambassador in Berlin on Friday following the arrest of a man reported to have spied for the United States, heightening friction between the two countries over US eavesdropping in Germany.
The US ambassador, John B Emerson, was called in "in connection with an investigation by the federal prosecutor," the German Foreign Ministry said in a statement. The US envoy "was asked to help in the swift clarification" of the case, it added.
Federal prosecutors say a 31-year-old German man was arrested on Wednesday on suspicion of spying for foreign intelligence services. They did not identify the suspect or the intelligence services.
German government spokesman Steffen Seibert said that Chancellor Angela Merkel had been informed of the arrest.
He declined to comment on reports by Der Spiegel magazine and the daily Sueddeutsche Zeitung that the man worked for Germany's foreign intelligence service, known by its German acronym BND.
The newspapers said the man was suspected of passing on information about a German parliamentary committee investigating the activities of US and foreign spy agencies in Germany.
He worked with US intelligence since 2012, they reported.
Seibert said members of the parliamentary panel had also been informed of the arrest.
If the media reports are confirmed then there can't just be a legal response, there also has to be a political response.
Reports that the National Security Agency spied on German citizens, including on Merkel's mobile phone, have caused friction between Berlin and Washington since they were first published last year, based on documents leaked by former NSA worker, Edward Snowden.
Martina Renner, a member of the opposition Left Party on the parliamentary panel, said the case indicated that anyone who examined Snowden's revelations in detail was subject to scrutiny by US intelligence agencies.
Her panel heard testimony Thursday from two former NSA employees, Thomas Drake and William Binney.
"If the media reports [about the case] are confirmed then there can't just be a legal response, there also has to be a political response," she said.
In his testimony, Drake said that cooperation between the NSA and Germany's BND greatly increased after the 9/11 attacks in the US. He described the German spy agency as an "appendage" of the NSA.
Seibert said Merkel discussed "foreign policy matters" in a telephone conversation with the US president, Barack Obama, late on Thursday. He said the conversation focused on Ukraine but wouldn't say whether the arrest was also discussed.
The US National Security Council declined to comment and BND did not immediately return a call seeking comment.