Germany has arrested an alleged spy accused of passing the US information from a committee looking into NSA activities.
It has heightened diplomatic tensions between the two countries following allegations in the Edward Snowden leaks that the US electronic spy agency tapped Angela Merkel’s phone along with wider surveillance of German citizens.
The German government has not denied reports by Der Spiegel and the Sueddeutsche Zeitung that the suspected spy was a double agent and worked for Germany's foreign intelligence service, the BND.
The newspapers said the man allegedly passed the US information about a German parliamentary committee’s investigation into the NSA’s activities.
He claimed to have worked with US intelligence since 2012, they reported.
A spokesman for the German government, Steffen Seibert, told reporters that the Chancellor and members of the parliamentary panel had been informed of the arrest.
Federal prosecutors confirmed a 31-year-old German man was arrested on Wednesday on suspicion of spying for foreign intelligence services.
They did not identify the suspect or say which agency he was suspected of working with.
Germany summoned the US ambassador in Berlin on Friday.
A statement from the foreign ministry said John B. Emerson was called in “in connection with an investigation by the federal prosecutor” and “was asked to help in the swift clarification” of the case.
The latest allegations of spying by the US, Germany’s supposed ally, have provoked outrage across the country, where memories of espionage by the Stasi secret police are still fresh for some residents.
Martina Renner, a member of the opposition Left Party on the NSA inquiry panel, said the case indicated that anyone who examined Snowden's revelations in detail was subject to scrutiny by US intelligence agencies.
“If the media reports are confirmed then there can't just be a legal response, there also has to be a political response,” she said.
Her panel heard testimony on Thursday from two former NSA employees, Thomas Drake and William Binney.
In his testimony, Mr Drake claimed that cooperation between the NSA and Germany's BND greatly increased after the 9/11 terror attacks in the United States.
He described the German spy agency as an “appendage” of the NSA.
Mr Seibert claimed Merkel discussed “foreign policy matters” in a telephone conversation with President Barack Obama on Thursday.
He said the conversation focused on Ukraine but would not say whether the arrest was also discussed.
Thomas Opperman, secretary of the Social Democratic Party group, said: “If the espionage allegations are proved, it is an outrageous attack on our parliamentary freedom.
“There is no justification for any power or country to enlist intelligence agents to spy on parliament.”
Katrin Göring-Eckardt, leader of Germany’s Green Party, told Spiegel Online: “All co-operation between German security services and allied agencies need to be reviewed.
“Should the suspected intelligence service spying on the parliamentary investigative committee be proved, it will be a huge fiasco for the BND and the government.”
The US National Security Council declined to comment.