US homeland security officials have defended heightened airport security screening measures but said they would consider adjustments to new rigorous pat-downs after complaints from travellers.

Homeland Security Secretary Janet Napolitano made it clear that new full-body scan checks would become the routine as hundreds of the machines are installed at US airports and that the alternative would be physical pat-downs.

'If there are adjustments we need to make to these procedures as we move forward, we have an open ear; we will listen,' she told reporters during a news conference at Ronald Reagan Washington National Airport.

'This is all being done as a process to make sure the travelling public is safe,' she said.

She said that the scans did not pose health risks and that privacy safeguards have been adopted to prevent the images from being saved or transmitted.

There are almost 400 body scan machines in some 68 US airports.

Some airports still only use metal detectors. Those who opt out of a body scan would be subject to a pat-down, which the Transportation Security Administration has made more rigorous in recent weeks and has provoked the backlash.

The DHS and its TSA have been scrambling to address a public backlash against the new security measures, including a call to boycott body scans on one of the busiest travel days, the day before Thanksgiving.

'I really regret that,' Ms Napolitano said of the proposed boycott.

'Our evaluation of the intelligence and risk indicated that we needed to move more quickly into the non-metal environment, to get liquids and powders and gels off of aircraft.'

TSA rushed deployment of body scanners after a foiled plot by a Nigerian man who tried to detonate explosives hidden in his underwear aboard a US flight from Amsterdam to Detroit. Last month authorities discovered explosives hidden in two packages aboard cargo flights to the US.

The Yemen-based militant group al-Qaeda in the Arabian Peninsula has claimed responsibility for the plots. Lawmakers are set to question TSA Administrator John Pistole about possible gaps in aviation security.

To stem the concerns, Ms Napolitano and Mr Pistole met last week with travel industry executives who have expressed worries that Americans will cancel their trips and thus hurt the fragile economy, which is still trying to recover from a recession.

Already the TSA has given a little ground after the flood of complaints, announcing that it has eliminated pat-downs for children under 12 and will develop alternative procedures for pilots who are already subject to extensive security checks.

'We've heard the concerns that have been expressed and agree that children under 12 should not receive that pat-down,' Mr Pistole said on NBC's 'Today Show'.

TSA had been reviewing the issue and Reuters last week reported on a father upset after his eight-year-old son was subjected to a pat-down.

TSA also is experimenting with some alternative checks for the pilots after their unions raised concerns about health risks of the scanners and objected to rigorous pat-downs.

DHS has said the scans involve less radiation than people receive otherwise on a daily basis.

Pilots' unions have said they already have gone through security background checks and have access to the cockpit, making further screening duplicative.

Ms Napolitano said she expected more details to resolve that issue soon.