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MAINZ, Germany -- Chancellor Angela Merkel broke her silence Monday on allegations that a German citizen had worked as a double-agent for the U.S.

"If the reports are correct it would be a serious case," Merkel told reporters during a news conference while visiting China. "It would be for me a clear contradiction to what I consider to be a trustful cooperation between agencies and partners."

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Her comments follow last week’s arrest of a 31-year-old employee of German intelligence service BND who is suspected of spying for U.S. agencies.

German-U.S. relations were been badly strained last year after Edward Snowden publicized the National Security Agency’s mass surveillance program in Germany, including the tapping of Merkel’s own mobile phone.

President Joachim Gauck added to growing chorus of criticism in Germany.

“If it really is the case that a service has been using an employee from our service in this way, we have to say: 'That is enough,'" he told German broadcaster ZDF.

"The NSA has a rather totalitarian understanding when it comes to surveillance methods"

According to German media reports, the BND suspect allegedly passed on more than 200 documents to U.S. agents since 2012, and received more than $34,000 in exchange.

Espionage is "not something we take lightly," government spokesman Steffen Seibert told reporters in Berlin on Friday.

“The NSA has a rather totalitarian understanding when it comes to surveillance methods and in this case it has completely ignored the political implications,” said Henning Riecke, head of the U.S.A. program at the Berlin-based German Council on Foreign Relations. “We are facing a crisis that is undermining transatlantic relations in a slow and continuous process."

Earlier this year, the country’s lower house of parliament formed an official inquiry committee, which is investigating the NSA’s activities in Germany.

Ahead of a trip to Washington in early May, Merkel had stressed that relations between the United States and Europe had been “severely shaken” and that trust between the two sides “needs to be rebuilt.” But following the two-day visit, Merkel came under heavy criticism for returning to Germany without a U.S. commitment to a so-called no-spy agreement, and of being too soft on the U.S.

Professor Michael Woffsohn of the German Armed Forces University in Munich downplayed the furor over the latest espionage allegations, calling them "naive."

“Surveillance is very common, even among allies,” he said. “And let’s not forget that Germany has been very dependent on American intelligence when it, for example, comes to jihadist terrorism.”