WASHINGTON (AFP) - Former computer technican Edward Snowden's blowing the lid on the United States' surveillance of phone calls and Web traffic has forced a tightening of security on system operators like him, the National Security Agency (NSA) said on Sunday.

NSA director Keith Alexander said it is overhauling its operations to keep a closer watch on contractors like the fugitive Snowden, who had top security clearance and "stole some of our secrets".

Mr Alexander, interviewed on ABC television, was not asked about Snowden's departure from his first haven Hong Kong for Moscow en route reportedly to a third nation.

Mr Alexander described Snowden as an NSA computer system adminstrator with top-secret security clearance who betrayed his nation by taking a trove of information from the NSA and fleeing from his base in Hawaii to Hong Kong.

No red flags went up to detect that theft, Mr Alexander said, and the NSA is working to overhaul things to prevent a repeat.

"Clearly, the system did not work as it should have," Mr Alexander said.

"We are now putting in place actions that would give us the ability to track our system administrators, what they doing, what they are taking," the army general said, adding that the NSA is implementing what he called a "two-man rule." He did not elaborate.

Also, he said: "We've changed the passwords. But at the end of the day we have to trust that our people are going to do the right thing."