President Barack Obama's spokesman, Robert Gibbs, said the government is "desperately" trying to balance procedures that maximize security and minimize invasiveness. He says the Transportation Security Administration procedures will continue to evolve.

"The evolution of the security will be done with the input of those who go through the security," Gibbs said.

Air travelers are protesting new requirements at some U.S. airports that they must pass through full-body scanners that produce a virtually naked image. The screener, who sits in a different location, does not see the face of the person being screened and does not know the traveler's identity.

Those who refuse to go through the scanners are subject to thorough pat-downs that include agency officials touching the clothed genital areas of passengers.

Earlier Monday, Transportation Security Administration chief John Pistole urged passengers angry over safety procedures not to boycott airport body scans.

He said he understands public concerns about privacy in the wake of the tough new security checks, but said that a relatively small proportion of the 34 million people who have flown since the new procedures went into effect have had body pat downs.

With the Thanksgiving travel rush less than 48 hours away, Pistole implored passengers Monday not to take delaying actions or engage in boycotts of body scans, actions he said would only serve to "tie up people who want to go home and see their loved ones."

Pistole had pledged Sunday to review security procedures in the wake of a public outcry. But he also said the TSA must balance people's demand for privacy with the need to protect passengers from those who would try to set off bombs on planes.

A loosely-organized Internet boycott of body scans is under way, and Pistole said he hoped people would exercise sound judgment over the busy Thanksgiving holiday. A National Opt-Out Day is scheduled for Wednesday to coincide with the busiest travel day of the year.

"Just one or two recalcitrant passengers at an airport is all it takes to cause huge delays," said Paul Ruden, a spokesman for the American Society of Travel Agents, which has warned its more than 8,000 members about delays resulting from the body-scanner boycott. "It doesn't take much to mess things up anyway — especially if someone purposely tries to mess it up."

Body scans take as little as 10 seconds, but people who decline the process must submit to a full pat-down, which takes much longer. That could cause a cascade of delays at dozens of major airports, including those in New York, Los Angeles, Chicago and Atlanta.

Pistole noted the alleged attempt by a Nigerian with explosives in his underwear to try to bring down an Amsterdam-to-Detroit flight last Christmas. "We all wish we lived in a world where security procedures at airports weren't necessary," he said, "but that just isn't the case."

The statement came just hours after Pistole, in a TV interview, said that while the full-body scans and pat-downs could be intrusive and uncomfortable, the high threat level required their use.

In the Sunday TV appearance, Pistole appeared to shrug off statements by Obama and Secretary of State Hillary Rodham Clinton that the TSA would look for ways to alter screening techniques that some passengers say are invasions of privacy.

Obama said in Lisbon on Saturday that he had asked TSA officials whether there's a less intrusive way to ensure travel safety. "I understand people's frustrations," he said, adding that he had told the TSA that "you have to constantly refine and measure whether what we're doing is the only way to assure the American people's safety."

Clinton, appearing Sunday on NBC's "Meet the Press," said she thought "everyone, including our security experts, are looking for ways to diminish the impact on the traveling public" and that "striking the right balance is what this is about."

She, for one, wouldn't like to submit to a security pat-down.

"Not if I could avoid it. No. I mean, who would?" Clinton told CBS' "Face the Nation."

"Clearly it's invasive, it's not comfortable," Pistole said of the scans and pat-downs during the interview on CNN's "State of the Union." But, he added, "if we are to detect terrorists, who have again proven innovative and creative in their design and implementation of bombs that are going to blow up airplanes and kill people, then we have to do something that prevents that."

Rep. John Mica, R-Fla., who is set to become Transportation Committee chairman when Republicans take over the House in January, differed with the approach.

"I don't think the rollout was good and the application is even worse. This does need to be refined. But he's saying it's the only tool and I believe that's wrong," Mica, a longtime critic of the TSA, said on CNN.

House Majority Leader Steny Hoyer, D-Md., appearing on CBS, said Congress would hold hearings on the "very controversial" issue of how to strike the right balance. Asked how he would feel about submitting to a pat-down, Hoyer said, "I don't think any of us feel that the discomfort and the delay is something that we like, but most people understand that we've got to keep airplanes safe."

Pistole was interviewed Monday morning on ABC's "Good Morning America," CBS's "The Early Show," NBC's "Today" show and MSNBC.