BERLIN — German-U.S. relations are facing a new test over a German intelligence employee who reportedly spied for the U.S., with Germany's president saying if the allegations are true, that kind of spying on allies must stop.

Prosecutors say a 31-year-old German was arrested last week on suspicion of spying for foreign intelligence services, and that he allegedly handed over 218 documents between 2012 and 2014. German media, without naming sources, have reported he was an employee of Germany's foreign intelligence service who says he sold his services to the U.S.

Germany's Foreign Ministry summoned the U.S. ambassador Friday to help clarify the case. The country's top security official stepped up the pressure Sunday.

"I expect everyone now to assist quickly in clearing up the accusations — and quick and clear statements, from the USA too," Interior Minister Thomas de Maiziere was quoted as saying in Bild newspaper.

The issue threatens to strain German-U.S. relations again after earlier reports that the National Security Agency spied on Germans, including on Chancellor Angela Merkel's cellphone.

If it turns out the U.S. "gave this kind of assignment to one of our intelligence employees, then it really has to be said: That's enough now," President Joachim Gauck said on ZDF television.

The head of a parliamentary committee investigating the activities of U.S. and allied spies, Patrick Sensburg, said he has no information that documents from the panel were spied on, but government documents destined for the committee may have been.

The U.S. Central Intelligence Agency and the National Security Council have declined to comment.

Former Secretary of State Hillary Rodham Clinton said at a book presentation in Berlin it's "a serious issue."

"Let's find out what the facts are and then let's act appropriately, but also try to be careful not to undermine the necessary cooperation which exists between us," she said.