The head of the National Security Agency Wednesday defended US surveillance programs as part of a "noble" mission to protect the nation and said reports on them were "sensationalized."

"The future of this country depends on our ability to defend against cyber attacks and terrorist threats, and we need the tools to do it," said General Keith Alexander, chief of the NSA.

The agency heads the PRISM program and other vast data collection efforts revealed in recent months by former US intelligence contractor Edward Snowden.

Alexander, speaking at the Billington Cybersecurity Summit in Washington, said there have been relatively few terror attacks on US soil since September 11, 2001 despite growing threats around the world.

"This is not by accident. It's by a lot of hard work," he told the forum. "Twenty-two cryptologists lost their lives in Iraq and Afghanistan. They're the heroes, not the (people behind) media leaks."

Alexander appealed to the public to support the surveillance effort, which is coming under scrutiny in Congress, and argued that the facts about the programs have been distorted.

"It's been sensationalized and inflamed in much of the reporting," he said.

"What's hyped up in a lot the reporting is that we are listening to your conversations, that we're reading your emails. That's not true... We understand our job is to defend this country. It's a noble mission."

Alexander repeated his assertion that more than 50 terrorist threats around the world have been foiled as a result of the intelligence gathered from the programs, which have been harshly criticized by US allies ranging from Germany to Brazil.

The thwarting of the attacks "would not have been possible without that capacity, and our allies have benefitted from that," he said.

The NSA has been in the center of a firestorm since the Snowden leaks which revealed wide-ranging programs which scoop up data on telephone calls and Internet activity.

On Snowden, Alexander did not mention him by name, simply calling him "the leaker," adding that "we trusted him and he betrayed our trust. That won't happen again. That doesn't make him a hero."

President Barack Obama has called on US intelligence agencies to release more classified documents to shed light on the spying effort, which he has defended as a legitimate bid to prevent terror attacks.

Alexander said US technology firms have been unfairly maligned in the reports.

These companies "are providing what the courts are directing them to provide. Our industry folks are taking a beating on this and it is wrong."

The comments came a day after stinging criticism of the programs from Senator Patrick Leahy, a Democrat who heads the chamber's judiciary committee.

Leahy told a Washington forum that the program allowing "bulk collection of Americans' phone records must end" and other programs must see closer scrutiny.

"Congress did not enact FISA (Foreign Intelligence Surveillance Act) in order to give the government dragnet surveillance powers that could sweep in the data of countless innocent Americans," Leahy said, according to a copy of his remarks on his website.