BERLIN — In the latest turn in the yearlong tensions with Germany over U.S. 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 U.S. 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 U.S. intelligence officials refused to comment on the arrest, one senior U.S. 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 midlevel 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 U.S. 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 U.S. 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 U.S. officials would say on the record whether the subject of the arrest came up during the call. But another senior U.S. 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.