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While razor blades and box cutters will still not permitted, passengers will be able to carry knives with folding blades 2.36 inches (6 centimetres) or shorter, as well as sporting goods such as golf clubs, hockey clubs, and novelty-sized baseball bats, Transportation Security Administration (TSA) head John Pistole said.
The changes will take effect on April 25 and will bring the United States into alignment with international rules.
Some security experts say security improvements since the 9/11 attacks -- most notably reinforced cockpit doors, better intelligence and motivated passengers -- have made the strict prohibition of small pen-knives unnecessary.
Pistole said the changes are in keeping with his "risk- based security" initiative and will permit airport screeners to focus on looking for bomb components, which present a greater threat to aircraft, instead of lesser threats.
But a union representing 90,000 flight attendants called the measure "a poor and short-sighted decision by the TSA."
"Continued prohibition of these items is an integral layer in making our aviation system secure and must remain in place," CNN quoted a statement by the Coalition of Flight Attendant Unions as saying.
Although it was widely reported that the 9/11 hijackers used "box-cutters" in their attack in 2001, the weapons were not recovered, and investigators believe other types of knives were used.
Immediately after the 9/11 attacks, the TSA prohibited a wide range of items from aircraft, but it has gradually been reintroducing some banned items such as cigarette lighters and matches. On occasion, such as a liquid bomb scare in August 2006, the TSA has expanded the list, banning large containers of liquids and gels.
Under the TSA's risk-based security, Pistole has sought to "reduce the haystack," giving modified screening to travellers under age 12 and over age 75. It has also expanded its TSA PreCheck initiative, which expedites checks of known passengers.
TSA spokesman David Castelveter said the changes announced on Tuesday will not slow down the screening process by requiring screeners to measure knife blades and weigh plastic bats. Screeners will use "common sense" when applying the rule, he was quoted as saying.
