At a news conference in Beijing, German Chancellor Angela Merkel was asked about allegations that a German man worked as a double agent for U.S. intelligence. (Reuters)

— An emerging scandal over a possible U.S. informant in the German intelligence service is threatening to spark a fresh rift between Washington and one of its closest allies over an explosive question here: Should friends spy on friends?

German politicians and media expressed mounting outrage Saturday, a day after reports in the German press that a 31-year old man arrested Thursday by German authorities was a mid-level member of German intelligence who passed state secrets to the United States. Authorities have not openly named the “foreign power” connected to the arrest, though the U.S. ambassador to Berlin, John B. Emerson, was summoned by the Foreign Ministry on Friday to offer “swift clarification” of the case.

The threat of a fresh spying scandal at a highly sensitive time in the U.S.-German relationship was already raising alarm on both sides of the Atlantic.

For months, the United States and Germany have sought to ease tensions after last year’s revelations that the U.S. National Security Agency had eavesdropped on the cellphone of German Chancellor Angela Merkel.

In response, German officials have privately and publicly shown their displeasure, using more than just words. Citing concerns over possible data disclosures, for instance, the German Interior Ministry said last month that it would not renew a contract with the U.S.-based firm Verizon.

File: German Chancellor Angela Merkel (Clemens Bilan/AFP/Getty Images)

Should the new investigation here conclude there was U.S. involvement, the consequences could be deeply damaging.

In recent months, Merkel has sought to lower tensions and German officials have acknowledged that their hopes to win a non-spying agreement with the United States is dead in the water — largely because Washington fears such a deal would set an unwelcome precedent among allies and rivals alike.

But a new instance of U.S. spying could seriously undermine attempts to mend strained ties just as the two governments are seeking to work together to check the ambitions of Russian President Vladimir Putin and defuse the crisis in Ukraine.

“When you have a dramatic crisis in Ukraine and the Middle East is going nuts, having no U.S.-German relationship is not an option,” said Ulrike Guerot, a Berlin-based political analyst and commentator. “But depending on what happens next, and the details of [this case], the governments need to recast the relationship.”

Guerot and others said it remained too soon to truly assess the possible damage since the facts of the case remained unclear. On Saturday, the U.S. Embassy in Berlin declined comment and German authorities were still withholding key details.

German news reports said the man under arrest had initially been suspected of spying for Russia. But during questioning, he reportedly told investigators that he had sold information to the United States.

Fueled by German news reports — including allegations the U.S. may have paid for information about the parliamentary committee investigating the NSA scandal — domestic anger already appeared to be growing.

German President Joachim Gauck told the ZDF broadcaster Saturday that if the reports were true, “One really has to say, enough is enough.”

Leading members of Merkel’s Christian Democratic Union (CDU), meanwhile, were describing the incident as serious.

Stephan Mayer, a national legislator from the CDU, told the Bild tabloid that “if it turns out to be true that the employee of the BND (Germany’s Federal Intelligence Agency) for years has been directed from the American embassy, this is a huge breach of trust in transatlantic relations. In a situation that’s already fragile, this spying incident would be another endurance test for German-American relations.”

Writing in the Süddeutsche Zeitung, columnist Stefan Kornelius struck a similar tone: “Should it be confirmed that an American intelligence service directed a BND-employee as a double agent, Germany and the U.S. will slide into a crisis of which, if one tried, would lack superlatives to describe.”

Stephanie Kirchner contributed to this report.