President Bush issued a broad defense yesterday of national intelligence efforts to combat terrorism in the United States and vowed that "the privacy of ordinary Americans is fiercely protected in all our activities."

Bush's comments in a hastily arranged White House appearance were prompted by USA Today's disclosure that the National Security Agency has secretly amassed call records of tens of millions of Americans obtained from AT&T, Verizon and BellSouth. Bush did not specifically mention the program, which does not involve listening to or recording conversations.

Amid a furor of protests over the clandestine effort from privacy advocates, members of Congress and others, Bush said the government isn't "mining or trolling through the personal lives of millions of innocent Americans."

But a current intelligence official and a former intelligence official familiar with how the program works said the NSA uses the call records to build a "spider web" of information that could involve extensive analysis of phone activity.

The patterns that are revealed help the agency better understand terrorist networks and identify suspected terrorist operatives or supporters, according to the two officials. They discussed it on condition of anonymity because the program is classified.

Begun shortly after the 9/11 terrorist attacks, the program uses an array of technical information about a phone call, including the numbers involved, time and date and duration.

The NSA puts the ever-growing database through what it calls "traffic analysis" to discern patterns of phone calling using powerful computer programs, the officials said.

The NSA's analysis sometimes starts with a specific telephone number and then examines all calls to and from that number. From there, the former intelligence official said, "You build a big spider web moving outward, looking at the calls to the original number, then the calls to and from those numbers and so on."

Several members of Congress briefed by the administration on the program confirmed it exists. The U.S. needs "to use modern technological tools" to defeat terrorists, Sen. Trent Lott, R-Miss., told reporters. He said the USA Today account undermines a "legitimate and legal" program.

Arlen Specter, R-Pa., chairman of the Senate Judiciary Committee, said he would summon phone company executives to testify about the program.