UPDATE 2-US has no plans to retreat from traveler patdowns

* Air security chief says risk requires tighter screening

* Heavy US holiday air travel season under way

* Senior lawmakers want review, hearings of TSA policy (Adds additional TSA data, statement)

By John Crawley

WASHINGTON, Nov 21 (Reuters) - U.S. homeland security officials have no plans to back away from airline passenger security patdowns despite traveler complaints that they violate constitutional rights and growing congressional concerns about the policy.

As the heavy holiday travel season got under way, John Pistole, head of the Transportation Security Administration, acknowledged on Sunday that the law enforcement-style patdowns, performed as an alternative or additional screening measure, can be unexpectedly intrusive.

"It's invasive; it's not comfortable," he told CNN's "State of the Union" program. TSA estimates that roughly less than 2 percent of the 2 million passengers screened daily, or some 40,000, are given the new patdowns.

But Pistole stressed that tighter security, including a wider use of controversial full body scanners at airports by year's end, is necessary to mitigate terrorism risks.

"I want to be as sensitive as I can to those folks. I'm very attuned given all the concerns that have been raised," Pistole said. But, "No, we're not changing the policies."

With pilots allowed to carry guns and cockpits hardened against hijacking threats following the 2001 hijack attacks on New York and Washington, screening in recent years has focused on potential bomb plots using sophisticated explosives that are hard to detect.

Obama administration officials have pointed to the thwarted bombing of U.S.-bound air cargo flights last month and last year's Christmas attempt to blow up a Delta Air Lines flight to Detroit with a bomb hidden in a passenger's clothes.

The Yemen-based group al Qaeda in the Arabian Peninsula claimed responsibility for both plots.

The issue has drawn the attention of President Barack Obama who said in Lisbon on Saturday that TSA is under "enormous pressure" after the Detroit incident to guard against a similar attempt. Obama said TSA has indicated that new procedures are "the only ones" they consider effective against that threat.

Obama said at a news conference that he is "constantly asking" whether the security approaches are absolutely necessary. "Have we thought it through? Are there other ways of accomplishing it that meet the same objectives?" he said.

Secretary of State Hillary Clinton said on Sunday she believes vigilance is necessary, but that "striking the right balance" between security and passenger interests requires precision.

"Let's not kid ourselves. The terrorists are adaptable," she said on NBC's "Meet the Press."

"You've got folks putting explosives in their underwear. Who would have thought that?" Clinton said referring to the accused Detroit bomber. But she acknowledged that she would not submit to a patdown if it were avoidable. "Who would?"

Already TSA has tweaked its patdown policy in recent days, agreeing to exclude children under 12 and pilots. Pistole issued a statement later on Sunday in which he said the TSA is constantly evaluating ways to adjust its screening methods.

"There is a continual process of refinement and adjustment to ensure that best practices are applied and that feedback and comment from the traveling public is taken into account," he said.

Consumer complaints about patdowns and revealing walk-through scanners have flooded airlines, congressional offices, civil liberties groups, and security agencies. Internet-fueled campaigns are urging travel boycotts.

Airlines are in the midst of a financial rebound and expect 24 million people to travel over the Thanksgiving holiday period. Airlines have passed along passenger concerns to TSA.

"We rely on TSA to do risk assessments and to implement appropriate security measures as they have done," said David Castelveter, a spokesman for the Air Transport Association, the industry's chief lobbying group.

Lawmakers from both parties have asked TSA to reconsider its new screening policies and more hearings are likely.

Patdowns are conducted now if passengers refuse to walk through a full-body screener, if an anomaly is found on the scan or if someone sets off traditional metal detectors.

"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," said House Democratic Majority Leader Steny Hoyer. (Additional reporting by Jeremy Pelofsky and Caren Bohan; Editing by Stacey Joyce)