Berlin: In the latest turn in the yearlong tensions with Germany over US spying, a German man was arrested this week on suspicion of passing secret documents to a foreign power, believed to be the United States. The US ambassador, John B. Emerson, was summoned to the Foreign Office here and urged to help with what German officials called a swift clarification of the case.

The arrest came as Washington and Berlin were trying to put to rest a year of strains over the National Security Agency’s monitoring of Germans’ electronic data, including Chancellor Angela Merkel’s cellphone, and just months after the collapse of an effort by Germany to strike a “no spy” accord with the White House.

While the White House and US intelligence officials refused to comment on the arrest, one senior US official said that reports in the German media that the 31-year-old man under arrest had been working for the United States for at least two years “threaten to undo all the repair work” the two sides have been trying to achieve.

The details of the latest case were murky. The media reports suggested that the man, a mid-level employee of the Federal Intelligence Service, was originally arrested on suspicion of spying for Russia. The Kremlin has markedly stepped up recruitment of German informants since the uprisings in Ukraine and the resulting sanctions aimed at Russia’s economy.

But according to the news reports and the account of the US official, the man told his interrogators he had been working for the United States for some time.

German news reports said that his work included reporting on the investigations into the NSA’s activities in Germany, which are the subject of a parliamentary inquiry, but the US official said he had no knowledge of whether that was the case. He spoke on the condition of anonymity to avoid complicating a diplomatically fragile intelligence issue.

The CIA and NSA both declined to comment on the allegations.

Merkel was informed of the case Thursday, her spokesman said, just before she spoke to President Barack Obama by telephone. But the White House described that conversation as one that was primarily about Ukraine and the continuing negotiations with Iran over its nuclear program. Neither German nor US officials would say on the record whether the subject of the arrest came up during the call. But another senior US official, speaking on condition of anonymity because the president’s conversations were intended to be private, said the issue did not come up on the call, which was previously scheduled to discuss other matters, and that Obama was not aware of the case at the time of the call.

If the man had been spying for the United States for two years, as the German news reports say, his recruitment would have predated the disclosures by Edward J. Snowden, the NSA contractor, of the long-running tapping of Merkel’s cellphone.

After the Snowden disclosures, Obama ordered a complete review of spying on allies and partners. In an interview last week, the new director of the NSA, Adm. Michael S. Rogers, said that review had resulted in the termination of a number of spying operations, not because they were illegal, but because they were unwise.

But in conversations with German officials over the past year, the Obama administration has made clear that its commitment extends only to Merkel herself and not other German officials. That was one of many sources of tension as the two countries, which traditionally share intelligence on terrorism suspects and nuclear proliferation, struggled and failed to reach a new accord.

The German Parliament is conducting an inquiry into the NSA’s activities in the country, and it heard its first testimony Thursday from two Americans who formerly worked for the agency. That testimony came hours after a 27-year-old student in Bavaria was identified by name as one of the spy agency’s surveillance targets, the first German other than Merkel to be named in that way.

The testimony Thursday lasted late into the evening, delayed in part by an extraordinary meeting between the inquiry panel and the control commission that oversees Germany’s intelligence services. The lawmakers were apparently informed of the arrest of the accused spy at that meeting; attendees at such sessions are sworn to secrecy.

Part of the hearing was conducted in closed session after one of the American witnesses, William E. Binney, said he would be discussing important secret information with the panel.

There was no immediate confirmation from the German government or the prosecutor’s office concerning the reports that the arrested man had been spying for the United States. A statement from the general prosecutor said he was detained Wednesday by officers from the federal criminal office, the most senior police authority in Germany. It did not give details about his occupation.

On Thursday, the suspect appeared before a federal court in Karlsruhe, where the federal prosecutor’s office is, and was ordered held “on urgent suspicion” of unauthorized intelligence activities, the prosecutor’s office said in a statement.

Hans-Christian Strobele, a member of Parliament from the Green Party who sits on both the intelligence oversight body and the NSA inquiry panel, said he had “no reason to deny” the published reports he had seen Friday about the spy case. But he and the head of the inquiry panel, Patrick Sensburg of the Christian Democratic party, each counselled caution.

Sensburg said by telephone that “some reports are simply false.” And Strobele, a veteran lawmaker who travelled to Moscow last fall to meet with Snowden, said Friday, “We must have patience and see whether information stands up to scrutiny.”

“It would be good to have a very quick reaction from across the Atlantic,” he added, though he noted that on the Fourth of July, Americans “have every reason to do something else.”