German Chancellor Angela Merkel said on Monday allegations that a German man worked as a double agent for U.S. intelligence were serious.

German Chancellor Angela Merkel said on Monday allegations that a German man worked as a double agent for U.S. intelligence were serious and, if true, were a clear contradiction of what cooperation between partners is supposed to be about.

The case risks further straining ties with Washington, which have been sorely tested by revelations last year of large-scale snooping on Germany by the U.S. National Security Agency (NSA).

"If the reports are correct it would be a serious case," Merkel told a news conference in Beijing, standing next to Chinese Premier Li Keqiang.

"If the allegations are true, it would be for me a clear contradiction as to what I consider to be trusting cooperation between agencies and partners."

The White House and State Department have so far declined to comment on the arrest of a 31-year-old employee of Germany's BND foreign intelligence agency.

According to intelligence and political sources, the man admits passing documents to a U.S. contact.

Those include information about a parliamentary committee looking into allegations by former U.S. intelligence contractor Edward Snowden that Washington carried out major surveillance in Germany, including monitoring Merkel's phone.

German Foreign Minister Frank-Walter Steinmeier, who was on a trip to Mongolia while Merkel was in China, said the spying case would have consequences if the circumstances are confirmed.

"We haven't finished clearing this up yet. But if suspicions are confirmed that American secret services were involved, it will become a political issue and we can't just get back to business as usual," he told reporters in Ulan Bator.