Lisbon, Portugal (CNN) -- President Barack Obama stood by new controversial screening measures Saturday, calling methods such as pat-downs and body scans necessary to assure airline safety.

Speaking at a NATO press conference in Lisbon, Portugal, the president called the balance between protecting travelers' rights and their security a "tough situation."

Per the new rules, travelers may be subject to full-body scans at 400 such machines in 69 airports nationwide. Those who voluntarily opt out -- as well as those who set off a scanning machine or a metal detector -- are subject to a pat-down. Some travelers have likened the pat-downs to groping.

The president said such methods are needed after what happened December 25, 2009, when Umar Farouk Abdulmutallab of Nigeria allegedly boarded a flight from Amsterdam to Detroit with a bomb hidden in his underwear. Abdulmutallab reportedly failed to set off the bomb, which metal detectors didn't detect, though his attempt led to airport screening procedures that have caused a holiday travel uproar.

"At this point, the Transportation Security Administration, in consultation with our counterterrorism experts, have indicated to me that the procedures that they've been putting in place are the only ones right now that they consider to be effective against the kind of threat that we saw in the Christmas Day bombing," said Obama.

TSA officers can use "professional discretion" to determine whether individuals should be subject to further screening, according to a statement from the federal agency. Critics have called the procedures invasive, with Rutherford Institute President John Whitehead saying the agency "is forcing travelers to consent to a virtual strip search or allow an unknown officer to literally place his or her hands in your pants."

The president, while noting that he didn't have personal experience with the new security measures, said he understands "people's frustrations. He said he's asked TSA for assurances that "what we're doing is the only way to assure the American people's safety." He said that he has also told the federal agency's administrators that they must consider whether there are "less intrusive" ways to obtain the same goals.

"One of the most frustrating aspects of this fight against terrorism is that it has created a whole security apparatus around us that causes huge inconvenience for all of us," he said.

Obama said that transportation security officials have a tough task, facing "enormous pressure" to prevent a terrorist attack.

In a public statement released Saturday, TSA Administrator John Pistole spelled out the new security measures and offered tips to those flying this Thanksgiving week.

"As you travel this holiday season, I want to remind you that TSA's mission is to ensure the safety of you the traveling public and we are committed to doing so efficiently, courteously and professionally," Pistole said.

In Portugal, the president vowed he'd try to find a way to make passengers feel more both comfortable and safe, whether it is through the current policies or with new ones.

"Every week I meet with my counterterrorism team and I'm constantly asking them whether -- is what we're doing absolutely necessary? Have we thought it through? Are there other ways of accomplishing it that meet the same objectives?" he said.