German Chancellor Angela Merkel has voiced concern about an alleged US spy in German intelligence, in her first comments on the affair.
Speaking on a visit to China, Ms Merkel said that if the allegations about a double agent were true, it would constitute a serious breach of trust.
“It would be a clear contradiction of what I consider to be trusting co-operation” with the US, she said.
She was speaking at a news conference with Chinese Premier Li Keqiang.
It is the second day of her three-day visit, which is dominated by trade issues.
Last week a German employee of the country’s foreign intelligence service (BND) was arrested.
The man is suspected of having handed over more than 200 documents over a two-year period in exchange for 25,000 euros (£20,000; $34,000).
“If the reports are correct, it would be a serious case,” Ms Merkel said on Monday.