Despite growing opposition from airlines and industry unions to the TSA's new policy allowing passengers to carry small knives on planes, the agency's head said he is not backing down.

"I have to make sure that TSA's focus is on those things that are most destructive to the aircraft," TSA administrator John Pistole told ABC News. "It is not pocket knives. It is those non-metallic improvised explosive devices, the bombs that are very small. They are concealable and they are well designed."

The new policy has drawn criticism and concerns from three major U.S. airlines, some pilots, flight attendants, federal air marshals, insurance companies and politicians.

The new policy, which will go into effect on April 25, will permit folding knives that do not lock and that have blades 2.36 inches or less in length and are less than 1/2-inch wide. Novelty-sized and toy bats less than 24 inches long, billiard cues, ski poles, hockey sticks, lacrosse sticks and two golf clubs will also now be allowed in carry-on luggage.

The change, which Pistole announced on March 5, came after a TSA working group recommended that such items were not a security threat, said ABC News. He reiterated that liquid explosives, meaning improvised explosive device or I.E.Ds, remain the "greatest concern."