The German government wants a quick and clear explanation from Washington for US intelligence’s apparent contact with a German man arrested last week on suspicion of being a double agent, the interior minister said in a newspaper interview.

“I expect everyone to cooperate promptly to clear up these allegations – with quick and clear comments from the United States as well,” Thomas de Maiziere told Bild newspaper, according to excerpts from tomorrow’s edition.

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, who admits passing documents to a US contact, according to intelligence and political sources.

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That includes information about a parliamentary committee looking into allegations by former US intelligence contractorEdward Snowden that Washington carried out major surveillance in Germany, including monitoring chancellor Angela Merkel’s phone.

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 US National Security Agency.

Surveillance is a sensitive issue in a country where the memory of the Nazi’s Gestapo secret police and communist East Germany’s Stasi means the right to privacy is treasured.

Head of state Joachim Gauck, a former Protestant pastor and rights campaigner in the old German Democratic Republic, told German TV the NSA affair was “a vexing episode”.

“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’,” the president said in a television interview to be broadcast later today.

Mr De Maiziere, one of the cabinet ministers closest to Ms Merkel, called it a “very serious case” which must be investigated fully to “gauge the scale of the alleged spying and especially answer the question of who was involved”.

The US ambassador was called in on Friday to hear Berlin’s request for an explanation and foreign minister Frank-Walter Steinmeier said today it was in Washington’s own interests to help with the “quickest possible clarification of the facts”.

It is not clear whether Ms Merkel discussed it with president Barack Obama in their phone call on Thursday but her spokesman Steffen Seibert said: “We don’t take the matter of spying for foreign intelligence agencies lightly.”

One lawmaker on the committee investigating the NSA affair said the man arrested had no direct contact with the committee, whose meetings are confidential, and was “not a top agent”.

The suspect had offered his services to the United States voluntarily, intelligence and political sources said, and had been paid about €25,000 ($34,100) for passing on 218 BND documents to his unidentified American contact.

After the Snowden revelations, Berlin demanded Washington agree to a “no-spy agreement” but the United States has been unwilling to make such a commitment. German officials also emphasise that they rely on intelligence from US agencies.

Former US secretary of state Hillary Clinton, who was in Berlin promoting her new book, said today it was “clearly a serious issue” but she hoped the affair would not “undermine the necessary cooperation which exists between us”.