Germany's domestic intelligence agency (BfV) has no evidence that the United States carried out industrial espionage in Europe, its chief said on Wednesday.

Allegations that Germany's BND foreign intelligence agency helped the U.S. National Security Agency (NSA) spy on European firms have put strains on Chancellor Angela Merkel's ruling coalition with the Social Democrats (SPD) and could damage relations with the United States.

Addressing a security conference in Berlin, BfV chief Hans-Georg Maassen warned against using every suspicion to discredit cooperation with Washington.

"To date we have no proof that American intelligence agencies are spying on top German companies," he said. "The Americans were, are and remain a very important partner for us."

Surveillance is an especially sensitive issue in Germany because of the extensive snooping by the Stasi secret police in old Communist East Germany and by the Gestapo in the Nazi era.

Revelations by former U.S. intelligence contractor Edward Snowden about wide-ranging NSA espionage in Germany caused public outrage when they first surfaced a couple of years ago.

Maassen said he had asked business groups for indications of Western espionage in light of the Snowden revelations, but he had yet to receive any such information.

He added that the existence of "selectors" - IP addresses, search terms and names - for defense companies did not mean that they were targets of industrial espionage in all cases.

Both foreign and German intelligence agencies, he said, have an interest in certain defense equipment not being delivered to countries like North Korea and that embargo regulations are kept.