Surveillance by the National Security Agency is undermining intelligence cooperation with allies as the U.S. fights the growing threat of Islamic extremists.

The June 2013 revelations of NSA spying by contractor Edward Snowden are having repercussions, particularly in Germany, even as many allies come to appreciate the need to keep closer tabs on potential terrorists in the wake of deadly attacks in Europe and North America.

Reports in the German media that the NSA asked the German intelligence service BND to spy on Siemens, a German company suspected of dealing with Russia, as well as other European companies and politicians, have rattled the government of Chancellor Angela Merkel, which is already dealing with demands from a parliamentary investigation into Snowden's allegations.

The BND last week reportedly stopped sharing Internet surveillance data with the NSA, the latest fallout from the scandal.

Efforts to smooth out the bumps caused by Snowden have contributed to some of the fallout, as European parliaments become more assertive at overseeing their own intelligence agencies, which often are full partners in the NSA's activities.

As European lawmakers have interacted with their U.S. counterparts, they have realized how much more control Congress exercises over the NSA and other intelligence agencies than they have over their own, said Rep. Robert Pittenger, R-N.C., who met last week with lawmakers from Germany and 23 other European nations in Vienna and Berlin.

"We have a lot of leverage on our intelligence community through the Congress," Pittenger said. "They didn't realize what we did and what we didn't do. Now they do."

As chairman of the Congressional Task Force on Terrorism and Unconventional Warfare, Pittenger has led efforts to repair the damage caused by Snowden's leaks to the media since they were first published in June 2013.

Deadly attacks in Europe and elsewhere over the past few months by Islamist extremists, such as the January killing of 17 people in France at the satirical magazine Charlie Hebdo and a Jewish market, have aided in the effort. The lower house of France's parliament last week overwhelmingly passed a sweeping surveillance measure dubbed the "French Patriot Act," after the 2001 law members of Congress are working to reauthorize, though it does not include bulk data collection measures such as those ruled illegal by a U.S. federal court.

Pittenger said the process has made lawmakers on both sides of the Atlantic more responsive to security and privacy needs in their efforts to craft a more sustainable response to the threat of terrorism.

"We want something that's going to survive," he said. "We shouldn't be knee-jerk."