BERLIN — With mystery enveloping a German intelligence-service employee accused of spying — reportedly for the United States — German officials and commentators Sunday angrily demanded a response from Washington, warning that an already troubled relationship was at risk of deteriorating to a new low.

The demands for a statement from the United States were nevertheless couched in cautious terms, suggesting that the scandal, which exploded Friday when Germany’s federal prosecutor reported the arrest of the 31-year-old employee of the Federal Intelligence Service, might not be as bad as initially feared. The chairman of a parliamentary inquiry into U.S. intelligence activities told German radio that it seemed there was no breach of security surrounding his committee’s work, as some news reports had suggested.

Still, the anger was palpable. President Joachim Gauck told German television that if it turned out that the United States had been spying on Germany, “then that is really a gamble with friendship, with a close alliance.”

“Then we really have to say, ‘Enough,’ ” Gauck added.

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Chancellor Angela Merkel, on a trip to China, kept silent, although reporters traveling with her cited unidentified people in her circle as saying she was “surprised” and “disappointed” at the suggestion that a U.S. intelligence agency had recruited a German agent. Foreign Minister Frank-Walter Steinmeier, traveling in Mongolia, was quoted in the German news media as saying that “if the reports are true, then we are not talking about something minor.”

The United States “should fully explain the matter as soon as possible,” Steinmeier added.

An American whom Germans did hear from during the weekend was Hillary Rodham Clinton, in Berlin to promote the German-language edition of her recent book, “Hard Choices,” about her years as secretary of state. While skirting judgment on the new espionage scandal, she emphasized at a Saturday reception at the ambassador’s residence that the relationship between the United States and Germany was valued and should not be “sidelined, downgraded or destroyed.”

At an appearance Sunday, she said, “Let’s find out what the facts are,” adding that relations “should not be put at risk.”

Over decades of friendship since 1945, sharp language like that of recent months has been rare in German-American dealings. Those ties have been strained since last summer, when documents leaked by Edward J. Snowden, a former National Security Agency contractor, suggested that the NSA had monitored the electronic data of millions of Germans. The German government then appeared to receive assurances from Washington that nothing illegal had occurred. But the subsequent revelation that Merkel’s own cellphone had been monitored stirred new anger in a nation that remembers the role played by a snooping state under the Nazis and the Communists.

The German government tried reaching a “no spy” accord with Washington, but the Obama administration rejected the idea. Months of attempts to restore relations have yielded the beginnings of a dialogue about how to deal with electronic data, cybersecurity and privacy.

Those talks, and much else, would be jeopardized if the Americans recruited an agent, German commentators warned.