Chancellor Angela Merkel says allegations that a German intelligence agent sold documents to the US risks undermining trust between the allies.
If the reports were correct it would be a "serious case", Merkel said at a press briefing with Premier Li Keqiang in Beijing yesterday.
"If the allegations are true, it would be for me a clear contradiction as to what I consider to be trusting cooperation between agencies and partners," she said.
United States officials have not commented on the arrest of an employee of Germany's BND foreign intelligence agency.
"We haven't finished clearing this up yet. But if suspicions are confirmed that American secret services were involved, it will become a political issue," he said in Ulan Bator.
The US ambassador to Germany, who was called to a meeting at the foreign ministry late on Friday, had been told Washington was expected to shed light on the reports "as quickly as possible", Steinmeier said.
Interior minister Thomas de Maiziere, in pre-released excerpts from yesterday's Bild newspaper, called for a "quick and clear" statement by the US on the allegations.
The employee of the German foreign intelligence agency arrested last week had been working for the CIA for close to two years, German media reported on Sunday.
"All signs indicate that he was acting for the Americans," the Frankfurter Allgemeine Sonntagszeitung newspaper quoted an unnamed senior official at Germany's foreign intelligence service as saying.
The weekly Bild am Sonntag newspaper, quoting information from security authorities, also said the man had worked for the CIA and handed over secret documents as recently as July 1.
Germany's federal prosecutor general confirmed a man was arrested last Wednesday on suspicion of acting for a foreign intelligence service, but did not specify which nation.
Both newspapers said the suspect had passed on two documents about a parliamentary panel established earlier this year to investigate the US NSA surveillance after revelations by former intelligence contractor Edward Snowden.
Agence France-Presse, Bloomberg