By Dan Merica
Editor's note: This is one in a series of stories and opinion pieces surrounding the Aspen Security Forum currently taking place in Aspen, Colorado. Security Clearance is a media sponsor of the event, which is taking place from July 17 to 20 in Aspen, Colorado.
The United States' antiterrorism chief is worried about the leaks that former government contractor Edward Snowden has carried out - particularly, he said Thursday, because our European allies are watching and reacting.
In a panel at the Aspen Security Forum, Matthew Olsen, director of the National Counterterrorism Center, said that while "it remains to be seen" how Snowden's leaks have affected relationships with U.S. allies, he is growing concerned.
"I an worried about it when I see what I read, particularly with respect to Europe and our European allies," he said. "How they may be reacting to this. But I think it just remains to be seen on that."
Olsen, who heads the center that is responsible for analyzing all terror threats, was noticeably measured in his answer.
His predecessor, Mike Leiter, who also participated in the panel, was far more blunt.
Leiter was director of the antiterrorism office during the Wikileaks situation, top-secret government cables were leaked to the Wikileaks website. Army Pvt. Bradley Manning, charged with the largest leak of classified documents in U.S. history, is now facing a court-martial trial.
"For an hour-long meeting, we easily had half an hour of that with me having my head bitten off and getting yelled at by our foreign service colleagues saying, 'How could you be so sloppy?'" Leiter said.
The former antiterrorism chief said reaction to Snowden could be worse.
"They are going to be much, much closer (holding onto) information that they collect," Leiter said. "It absolutely undermines those relationships."
Snowden, who is currently a man without a country in the transit zone of Moscow's airport, is seeking asylum from a number of countries. After leaking top-secret documents to the media, the former government contractor fled to Hong Kong and then to Russia. Venezuela, Bolivia and Nicaragua have offered Snowden asylum.
In late June, the German magazine Der Spiegel reported that - according to Snowden's leaks - the United States National Security Agency had tapped European allies. In particular, the report said that the United States had bugged European Union offices in Washington and New York, and conducted an "electronic eavesdropping operation" that tapped into a EU building in Brussels, Belgium.
The magazine's report also said that NSA spying has targeted telephone and Internet connection data in Germany more than any other European nation. An average of up to 20 million phone connections and 10 million Internet data connections are surveyed daily, Der Spiegel said, noting that the intensity of surveillance puts the U.S. ally on par with China, Iraq and Saudi Arabia.
A number of European leaders said they were appalled by the reported U.S. program and the Der Spiegel story.
"I am deeply worried and shocked about the allegations," European Parliament President Martin Schulz said in a statement. "If the allegations prove to be true, it would be an extremely serious matter which will have a severe impact on EU-U.S. relations."
Leaders in France and Germany conveyed similar feelings.
"We have a strategic problem and our strategic problem is we can't keep our national secrets secret," Leiter concluded. "Those things that we should be protecting, we are not effectively protecting. That hurts our partners, whether it is our corporate partners, foreign partners, and it helps out enemies."
- CNN's Josh Levs and Catherine E. Shoichet contributed to this report.