BERLIN — With mystery enveloping a German intelligence service employee accused of spying — reportedly for the United States — German officials and commentators on 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 on 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 American 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, whose role is largely ceremonial but who increasingly speaks out on daily matters, 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,' ” Mr. Gauck added.

Chancellor Angela Merkel, on a trip to China but under pressure to address the issue, particularly after the president spoke out so strongly, broke with her usual practice of not commenting on domestic affairs while traveling. “If the allegations are true, it would be for me a clear contradiction as to what I consider trusting cooperation between agencies and partners,” she said in Beijing on Monday.