James Clapper reportedly warned in a secret directive that it was no longer possible to rely on Germany to protect classified documents, according to a US intelligence official quoted by the mass-circulation newspaper "Bild" on Saturday.

A German parliamentary committee is currently investigating allegations that the country's foreign intelligence agency, the BND, helped the US National Security Agency (NSA) spy on European companies and government officials.

"Bild" said Clapper had expressed concern in an internal document about the possibility of classified US material being passed from the investigative panel to the media, posing a danger to US interests.

"What the German government is staging there is more dangerous than the Snowden revelations," the document said.

Causing particular unease are lists of so-called "selectors," or search criteria, the NSA had asked the German agency to use in surveillance activities conducted from the BND's facility at Bad Aibling.

German opposition lawmakers in the parliamentary committee have called on the government to publish the NSA lists, which may include data such as names, telephone numbers, or Internet Protocol (IP) addresses.

Scaled back partnership?

Schindler: 'We need international cooperation.'

The BND facility in southern Germany was meant to gather data coming out of Somalia and Afghanistan, but in recent weeks it came to light that some of those selectors had helped the NSA spy on targets in Europe.

According to an official cited in the "Bild" report, Clapper mentioned the possibility of surveillance activities, like those carried out at Bad Aibling, being transferred to other friendly services. He also said US services had already put a stop to some joint projects with the BND, and should look into areas where cooperation could be scaled back or terminated.

When asked for comment, a spokesman for the National Security Council of the United States told "Bild" the parliamentary inquiry was "an internal matter for Germany."

'The NSA is our partner'

Earlier in the week, BND head Gerhard Schindler told the inquiry such probes into the alleged wrongdoing of the BND could threaten intelligence cooperation with allies who feared having their practices exposed.

"These developments cause me great worry, as they will ultimately call into question the ability of this service to function in the future," he told lawmakers. "We cannot do our job without international cooperation."

Schindler acknowledged that while his agency had failed to properly check the list of targets from the NSA, cooperation between the two groups was vital for Germany's security.

The NSA has been widely criticized in Germany since revelations leaked in 2013 by its former contractor Edward Snowden showed some of the extent of the US agency's activities, including tapping Chancellor Angela Merkel's phone.

nm/sms (dpa, AFP, AP)