General Keith Alexander, director of the American National Security Agency (NSA), defended the bulk gathering of metadata on phone calls, saying, "There is no other way that we know of to connect the dots."
The NSA collects information about which phone numbers are involved and the duration of calls, but not any information from actual conversations.
He told senators at a hearing that ending the mass collection program "is absolutely not an option".
Members of the US Congress have called for greater restraints on US spying programs after media reports in recent months revealed the mass surveillance of telephone and internet records, and spying on international leaders' mobile communications.
Former government contractor Edward Snowden began providing information about the extent of US spying efforts to the media earlier this year, prompting outrage from US allies and civil libertarians.
US President Barack Obama has said that the US is to unveil "self restraints" on surveillance in the coming weeks after an internal review.