A civil liberties group branded the program, authorized by a top secret court order, as "beyond Orwellian" but a top Republican lawmaker said it had directly thwarted a terror attack in the United States in recent years.
The controversy looked set to widen as the Washington Post and Britain's Guardian newspaper reported that the National Security Agency (NSA) had tapped directly into the servers of Internet giants -- including Microsoft, Google, Facebook and Apple -- to obtain videos, photographs and emails.
Advocates say the data, collected on calls inside and outside the United States by the NSA, can be crunched to show patterns of communication to alert spy agencies to possible planning for terror attacks.
Senior US officials, while not confirming reports in the Guardian, defended the concept of collecting millions of phone records, and argued the program was lawful and subject to multiple checks and balances across the government.
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"What we need to do is balance that priority with the need to protect civil liberties," he said, adding that President Barack Obama welcomed public debate on the issue.
Mike Rogers, Republican chairman of the House Permanent Select Committee on Intelligence, said the program was vitally important.
"Within the last few years this program was used to stop a terrorist attack in the United States. We know that," Rogers said.
A US official said the program allows counterterrorism investigators to find out whether suspected terrorists have been in contact with other suspects, particularly people located in the United States.
Randy Milch, Verizon's Executive Vice President and General Counsel, said in a message to staff he was legally forbidden to comment but that any such court order would compel the company to comply.